Compact-type metal halide discharge lamp

ABSTRACT

A metal halide discharge lamp which essentially permits disusing mercury is provided. The metal halide discharge lamp comprises a refractory and transparent hermetic vessel, a pair of electrodes fixed to the hermetic vessel, and a discharge medium sealed in the hermetic vessel and containing a first halide, a second halide, and a rare gas. The first halide is a halide of a metal which achieves a desired light emission. The second halide has a relatively high vapor pressure, being at least one halide of a metal which is unlikely to emit a visible light compared with the metal of the first halide and acts as a buffer gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application No. 09/087,311, filedon May 29, 1998, now U.S. Pat. No. 6,353,289 B1 the benefit of thefiling date of which is hereby claimed under 35 U.S.C. §120, which inturn claims the benefit of the following Japanese applications, thebenefit of the filing date of which is hereby claimed under 35 U.S.C.§119:

Application No. 9-148994, filed in Japan on Jun. 6, 1997

Application No. 9-311833, filed in Japan on Nov. 13, 1997

Application No. 9-346033, filed in Japan on Dec. 16, 1997

Application No. 9-346035, filed in Japan on Dec. 16, 1997

Application No. 10-028134, filed in Japan on Feb. 10, 1998

BACKGROUND OF THE INVENTION

The present invention relates to a metal halide discharge lamp, alighting device for a metal halide discharge lamp, and an illuminatingapparatus using a metal halide discharge lamp.

A metal halide discharge lamp comprises a light-emitting tube providedwith a pair of electrodes arranged to face each other. A rare gas, ahalide of a light-emitting metal, and mercury are sealed in thelight-emitting tube to form the metal halide discharge lamp. Thedischarge lamp of the particular construction exhibits a relatively highefficiency and high color rendering properties and, thus, is usedwidely.

The metal halide discharge lamp is classified into a short arc type anda long arc type. The short arc type metal halide discharge lamp is usedin projectors such as a liquid crystal projector in which light raysemitted from a lamp are collected so as to be projected onto a screen,and an overhead projector, and is also used for illumination of shops inthe form of down light and spot light. Also, a small short arc typemetal halide discharge lamp has come to be used in recent years as aheadlamp of a vehicle in place of a halogen lamp.

As described in, for example, Japanese Patent Disclosure (Kokai) No.2-7347, it is absolutely necessary to seal about 2 to 15 mg of mercuryin a metal halide discharge lamp used as a headlamp of a vehicle.

On the other hand, Japanese Patent Disclosure No. 3-112045 discloses ametal halide discharge lamp which does not necessitate the sealing ofmercury. In this prior art, a rare gas such as helium or neon is sealedin the lamp at a pressure of 100 to 300 Torr in place of mercury so asto obtain a desired lamp voltage. Since the atom of each of these raregases has a small radius, the rare gas permeates through a quartz glassand, thus, the hermetic vessel of the lamp is formed of a transparentceramic material.

On the other hand, the long arc type metal halide discharge lamp is usedmainly for the general illumination purposes. For example, the dischargelamp of this type is used as an illumination equipment for a highceiling, a light projector, a street lamp and as an illuminationequipment for roads. Further, a metal halide discharge lamp whichgenerates ultraviolet rays is used for the manufacture of aphoto-setting synthetic resin or ink. The metal halide discharge lampused for this purpose is also of a long arc type.

In any of the short arc type and long arc type metal halide dischargelamps which have been put to practical use nowadays, it is absolutelynecessary to use mercury, because, in the metal halide discharge lamp,mercury serves to obtain a desired lamp voltage so as to maintainsatisfactory electric properties.

To be more specific, where, for example, the lamp voltage is unduly low,the lamp current must be increased in order to obtain a desired lampinput. In this case, problems are brought about that the currentcapacities are increased in the related facilities such as the lightingdevice, illuminating device and wiring. Also increased is the heatgeneration.

On the other hand, where the lamp current is unduly high, the electrodeloss is increased, leading to a low lamp efficiency. To be morespecific, the electrode drop of the metal halide discharge lamp isconstant for each lamp. As a result, if the lamp voltage is unduly low,the lamp current must be increased for making up for the low lampvoltage, with the result that the electrode loss is increased inproportion to the lamp current so as to lower the lamp efficiency.

As pointed out above, it is generally advantageous in a discharge lampto set the lamp voltage at a value as close to the input voltage of thelamp as possible, i.e., as high as possible, as far as the arc does notdisappear.

Let us describe the reason why the mercury sealing was required in theconventional metal halide discharge lamp while giving consideration tothe lamp voltage with reference to FIG. 1. As shown in the drawing, thelamp comprises a hermetic vessel 1, a pair of electrodes 2, 2, and leadwires 3, 3. The lamp voltage V1, which denotes the voltage between thelead wires 3, 3 when the metal halide discharge lamp is lit, can berepresented by formula (1) given below:

V 1=E×L+Vd  (1)

where E is a degree of potential inclination of the plasma between theelectrodes, L is a distance between the electrodes, and Vd is anelectrode drop.

The potential inclination degree E of the plasma can be represented byformula (2) given below:

E=I/2π∫σrdr  (2)

where I is a lamp current, σ is an electrical conductivity of theplasma, which is a function of temperature T, and r is a distance of anoptional point from the center in the radial direction.

If a substance A is supposed to be present within the discharge spaceduring the lighting of the metal halide discharge lamp, the electricalconductivity σ of the substance A at temperature T is given by formula(3) below:

σ=C·N _(E)/(T ^(½)·(N _(A) ·Q))  (3)

where C is a constant, N_(E) is an electron density, N_(A) is a densityof the substance A, and Q is a cross section of impingement of theelectron against the substance A.

As apparent from formula (1), the lamp voltage V1 is increased withincrease in the potential inclination degree E and with increase in thedistance L between the electrodes. On the other hand, formula (2)indicates that the potential inclination degree E is increased withdecrease in the electrical conductivity σ and with increase in the lampcurrent I. Further, formula (3) indicates that the electricalconductivity u is decreased with decrease in the electron density N_(E)and with increase in the density N_(A) of the substance A and in theimpinging cross section Q. It follows that, where the distance L betweenthe electrodes and the lamp current I are set constant, the conditionsof the substance A, under which the lamp voltage V1 is increased, arethat the substance A is unlikely to be ionized to diminish the valueN_(E), that the substance A has a high density within the lamp toincrease the value N_(A), and that substance A has a large cross sectionQ of the electron impingement.

It should be noted that mercury has a very high vapor pressure, i.e., 1atmosphere at 361° C., is unlikely to be ionized, and has a large crosssection of the electron impingement. It follows that a desired lampvoltage can be easily obtained by controlling the sealing amount ofmercury in accordance with the size of the lamp. In other words, mercuryis sealed in the conventional metal halide discharge lamp because adesired lamp voltage can be obtained easily.

It should be noted in this connection that, in the case of a metalhalide discharge lamp, it is necessary to set the mercury vapor pressurehigher with miniaturization of the lamp in which the distance L betweenthe electrodes is shortened in order to ensure a desired lamp voltage.For example, in a small short arc type metal halide discharge lamp whoselight-emitting tube has an inner volume of 1 cc or less, the mercuryvapor pressure during lighting of the lamp is as high as at least 20atmospheres.

Let us describe the problems which are brought about where mercury issealed in a metal halide discharge lamp and the problems which arebrought about where mercury is not sealed in the conventional metalhalide discharge lamp.

Problems Brought about by Mercury Sealing

Air pollution and water contamination problems attract worldwideattentions nowadays. Since mercury is harmful to the health of the humanbeing, it is naturally desirable to decrease the amount of mercury usedor not to use mercury at all in the field of illumination. In otherwords, the greatest problem inherent in the conventional metal halidedischarge lamp is that mercury is sealed in the lamp.

In addition, many problems remain unsolved when it comes to the metalhalide discharge lamp in which mercury is sealed for obtaining a desiredlamp voltage, as pointed out below:

1. Poor in Rising of Spectral Characteristics in the Start-up Time:

Where a metal halide discharge lamp is used in the headlamp of avehicle, required is the instant rising of the light flux. To meet thisrequirement, employed is a lighting system in which xenon is sealed as astarting gas at a high pressure, and a large current is allowed to flowin the initial period of the lighting, followed by gradually decreasingthe current with time. It is certainly possible to achieve the instantrising of light flux in this fashion. However, since mercury is rapidlyevaporated in the switch-on time, mercury takes much energy so as tocause delay in the rising of the vapor pressure of a light-emittingmetal. It follows that mercury continues to emit light of high intensityfor 10 to 20 seconds. It should be noted that the light emitted frommercury is poor in color characteristics and in color renderingproperties. Also, the chromaticity of the light emitted from mercuryfails to fall within a range of whiteness. Since the rising of thespectral characteristics is very poor as described above, it takes along time to obtain emission of light having desired spectralcharacteristics.

2. Unsuitable for Light Control (Dimming):

A change in temperature of the light emitting tube brings about a greatchange in the color temperature of the emitted light and, thus, in thecolor rendering properties, as apparent from FIG. 2. Specifically, FIG.2 is a graph showing the distribution of an emission spectral of aconventional short arc type metal halide discharge lamp for projection.The wavelength (nm) is plotted on the abscissa of the graph, with therelative emission power (%) being plotted on the ordinate.

Sealed in the conventional short arc type metal halide discharge lampare 6.65×10⁴ Pa of argon as a rare gas, 1 mg of dysprosium iodide (DyI₃)as a halide, 1 mg of neodymium iodide (NdI₃) as a halide, and 13 mg ofmercury. The emission spectral consists of a continuous light emissioncaused by dysprosium and neodymium and main bright-line spectra causedby the elements given above the arrows in the drawing. As seen from thegraph, the bright-line spectrum caused by mercury has a large power.

It should be noted that the amount of light emission from each of thelight-emitting metals is changed proportionally to the vapor pressurewithin the lamp. Since the vapor pressure of a halide of alight-emitting metal is markedly lower than that of mercury, a change intemperature of the light emitting tube causes a change in theevaporation amount of the halide, leading to a change in the vaporpressure within the lamp. As a result, the amount of light emitted fromthe light-emitting metal is also changed.

On the other hand, the vapor pressure of mercury is so high that achange in temperature of the light emitting tube does not bring about anappreciable change in the mercury vapor pressure, leading to a smallchange in the amount of light caused by the strong bright-line spectrumof mercury. It follows that, if the input power supplied to thelight-emitting tube is decreased, the light emission caused by mercuryis rendered relatively predominant. As a result, the color temperatureof the emitted light is lowered, leading to poor color renderingproperties. This indicates that the conventional metal halide dischargelamp, which requires mercury sealing, is unsuitable for the lightcontrol (dimming).

In the case of a headlamp for a vehicle, dimming is required for thelighting in the day time (day light) employed in the U.S.A. and Europe.However, the color characteristics are markedly impaired in theconventional metal halide discharge lamp requiring the mercury sealing.

3. Large Unevenness in Properties:

The metal halide discharge lamps having mercury sealed therein areuneven in temperature of the light emitting tubes, which are caused byunevenness in the size of the individual lamps. As a result, unevennessin the characteristics tends to be brought about even under the sameinput power. Also, the characteristics are likely to be changed by thetemperature elevation in the coolest region caused by the blackening ofthe light-emitting tube used over a long period of time. Thesedifficulties tend to bring about a problem particularly where aplurality of metal halide discharge lamps are used in combination forillumination as in, for example, shops.

4. Difficult to Re-start up Instantly:

As described previously, the distance between the paired electrodes issmall in a short arc type small metal halide discharge lamp, making itnecessary to set the mercury vapor pressure at a high value.Specifically, the mercury vapor pressure is set at such a high value as,for example, at least 20 atmospheres.

Further, when used in a headlamp for a vehicle, xenon is also sealed inthe lamp at a high pressure. For example, the xenon pressure is as highas about 35 atmospheres during the lighting. Since the mercury vaporpressure and the xenon vapor pressure are very high during the lighting,it is necessary to apply a pulse voltage of a very large power in there-start up time. It follows that the lighting circuit is renderedcostly. In addition, it is necessary to insulate the circuit, the lampand the equipment housing them against a high voltage.

5. Rupture of Light-Emitting Tube

Since the mercury vapor pressure is very high during the lighting asdescribed previously, strain of the lamp is increased during lighting ofthe lamp over a long period of time, with the result that the lamp tendsto be ruptured. The problem of rupture markedly lowers the reliabilityof the lamp.

6. Low Screen Brightness when used in Projector:

Where a short arc type metal halide discharge lamp is used in aprojector in which the light emitted from the lamp used as a lightsource is collected through an optical system such as a liquid crystalprojector so as to illuminate, for example, a screen arranged apart fromthe projector, it is of high importance to suppress the loss of thelight emitted from the discharge lamp when the emitted light passesthrough the optical system so as to permit the emitted light to arriveat the screen as much as possible. In order to improve the brightness ofthe screen by suppressing the light loss, it is necessary for the arc ofthe discharge lamp to be narrowed thin. The expression “narrow arc”denotes that the arc temperature distribution is sharp.

It should be noted in this connection that the light emitted frommercury is absorbed and, thus, is optically thick. Since energy isabsorbed by the absorption of the light emitted in the intermediate andlow temperature regions, the temperature is elevated. As a result, thearc temperature is distributed to depict a parabola, making it difficultto narrow the arc. On the other hand, it is known to the art that, ifthe light emission is very much increased by using scandium or a rareearth metal as a light-emitting metal, the arc can be narrowed even inthe presence of mercury. In this case, however, convection occursvigorously if the lighting pressure of mercury is high, with the resultthat the arc is rendered unstable. It follows that it is impossible toput this technique to practical use.

Problems Inherent in the Conventional Lamp which does not necessitateMercury Sealing

In the metal halide discharge lamp which does not necessitate mercurysealing, the partial pressure of helium or neon within thelight-emitting tube is markedly increased during the lighting. If thelight-emitting tube is constructed to withstand the high pressure, it iscertainly possible to obtain a metal halide discharge lamp in whichmercury is not sealed. The possibility itself of obtaining a metalhalide discharge lamp not requiring mercury sealing is worthy of afavorable evaluation. However, it is practically difficult to permit ametal halide discharge lamp of the construction similar to that of theconventional lamp to withstand the high pressure within the lamp duringthe lighting of the lamp. Where, for example, a lamp voltage of 50 to60V is required in s small metal halide discharge lamp, the pressure ofhelium or neon within the lamp is expected to exceed 150 atmospheresduring the lighting of the lamp. It follows that the hermetic vesselwidely used in the conventional lamp fails to obtain a high reliabilityin respect of the measure against rupture of the hermetic vessel.

BRIEF SUMMARY OF THE INVENTION

A main object of the present invention is to provide a metal halidedischarge lamp which essentially permits disusing mercury, which isharmful to the living environment of the human being, and which alsopermits obtaining electrical characteristics and light-emittingcharacteristics substantially equal to those produced by a metal halidedischarge lamp in which is sealed mercury, to provide a lighting devicefor the particular metal halide discharge lamp, and to provide anillumination apparatus using the particular metal halide discharge lamp.

An auxiliary object of the present invention is to provide a metalhalide discharge lamp which permits a good rising of chromaticity at thestart-up time, which permits light control for the dimming purpose,which is small in unevenness of the characteristics, which permits aninstant re-start up easily, and which is effective for preventing thehermetic vessel from being ruptured, to provide a lighting device forthe particular metal halide discharge lamp, and to provide anillumination apparatus using the particular metal halide discharge lamp.

Another auxiliary object of the present invention is to provide a metalhalide discharge lamp in which mercury is not sealed, which is small inheat loss, and which is effective for preventing the light emittingefficiency from being lowered, to provide a lighting device for theparticular metal halide discharge lamp, and to provide an illuminationapparatus using the particular metal halide discharge lamp.

Another auxiliary object of the present invention is to provide a metalhalide discharge lamp which is lit by a DC current to emit light freefrom color difference and color separation, and which permits thelighting circuit to be manufactured as a low cost, to provide a lightingdevice for the particular metal halide discharge lamp, and to provide anillumination apparatus using the particular metal halide discharge lamp.

Another auxiliary object of the present invention is to provide a metalhalide discharge lamp which is adapted for use as a headlamp for avehicle such as an automobile, to provide a lighting device for theparticular metal halide discharge lamp, and to provide an illuminationapparatus using the particular metal halide discharge lamp.

Still another auxiliary object of the present invention is to provide apractical metal halide discharge lamp which is effective for preventinga hermetic vessel from being ruptured during the lighting of the lampeven if the mechanical strength of the hermetic vessel is substantiallyequal to that of the conventional hermetic vessel in which is sealedmercury, to provide a lighting device for the particular metal halidedischarge lamp, and to provide an illumination apparatus using theparticular metal halide discharge lamp.

According to a first aspect of the present invention, there is provideda metal halide discharge lamp which essentially permits disusingmercury, comprising:

a refractory and transparent hermetic vessel;

a pair of electrodes fixed to the hermetic vessel; and

a discharge medium sealed in the hermetic vessel and containing a firsthalide, a second halide and a rare gas, the first halide being a halideof a metal which achieves a desired light emission, the second halidehaving a relatively high vapor pressure, being at least one halide of ametal which is unlikely to emit a visible light compared with the metalof the first halide, and acting as a buffer gas.

The terms used in the present specification are defined to denote thetechnical meanings described below unless otherwise specifiedparticularly:

Hermetic Vessel . . . The refractory and transparent hermetic vesselused in the present invention is formed of a material capable of fullywithstanding the ordinary operating temperature of a discharge lamp andalso capable of leading the visible light of a desired wavelength regionemitted by the discharge to the outside. Any material can be used forforming the hermetic vessel as far as the requirements given above aremet. For example, it is possible to use quartz glass, or ceramicmaterial such as transparent alumina or YAG as well as single crystalsthereof.

Incidentally, it is acceptable in the present invention to form on theinner surface of the hermetic vessel a transparent film resistant tohalogen and to metals, as required. It is also acceptable to modify theinner surface of the hermetic vessel, as required.

Electrode . . . The metal halide discharge lamp of the present inventioncan be constructed so as to be lit by either an AC current or a DCcurrent. Therefore, where the lamp is operated by an AC current, thepaired electrodes are of the same construction. However, where the lampis operated by a DC current, the anode whose temperature is elevatedseverely in general is allowed to have a heat radiating area larger thanthat of the cathode.

The metal halide discharge lamp of the present invention may be eitherof a short arc type or of a long arc type. The short arc type dischargelamp is of a so-called electrode stability type, in which the distancebetween the electrodes arranged within the hermetic vessel is diminishedso as to allow the electrodes to stabilize the arc discharge. Therefore,the light emission from the discharge lamp can be made as close to thatof a dot light source as possible, making it possible to allow theoptical system such as a light reflector or a lens to collect lightefficiently. A small short arc type metal halide discharge lamp is usedin a projector such as a liquid crystal projector or in a headlamp for avehicle such as an automobile. In this case, a suitable distance betweenthe electrodes of the metal halide discharge lamp is practically at most6 mm. If the distance between the electrodes exceeds 6 mm, the lightemission from the discharge lamp is rendered widely different from thatfrom a dot light source, leading to poor focusing characteristics of theoptical system. Where, for example, the discharge lamp, in which thedistance between the electrodes exceeds 6 mm, is used as a light sourceof a liquid crystal projector, the screen brightness is lowered.

Under the circumstances, the small short arc type metal halide dischargelamp of the present invention is defined to denote a discharge lamp inwhich the distance between the electrodes is at most 6 mm. Preferably,the distance between the electrodes should be at most 5 mm. Further,where the discharge lamp is used in a projector such as a liquid crystalprojector, the distance between the electrodes should be 1 to 3 mm.Incidentally, the distance in question represents the distance betweenthe tips of the electrodes.

On the other hand, the long arc type metal halide discharge lamp of thepresent invention is of a so-called tube wall stability type, in whichthe distance between the electrodes arranged within the hermetic vesselis set longer than the inner diameter of the hermetic vessel so as toallow the arc discharge to be stabilized by the inner surface of thehermetic vessel. In general, the long arc type metal halide dischargelamp is widely used for the illumination purpose.

Discharge Medium . . . As described previously, the discharge mediumused in the present invention consists essentially of a first halide, asecond halide and a rare gas.

The first halide is a halide of a metal which emits a desired light suchas a visible light or an ultraviolet light. Where a halide of a metalwhich efficiently emits a visible light is used as the first halide inorder to utilize the visible light, the vapor pressure of the firsthalide is not necessarily high in general during the lighting of thelamp.

The second halide, which is also a halide of a metal, should have arelatively high vapor pressure during the lighting of the lamp. Themetal contained in the second halide is not particularly limited as faras the metal is unlikely to emit a visible light compared with the metalcontained in the first halide. The expression “relatively high vaporpressure” denotes that the vapor pressure need not be as high as thevapor pressure of mercury. Preferably, the pressure within the hermeticvessel during the lighting of the lamp should be about 5 atmospheres orless. During operation of the lamp, it is desirable for the vaporpressure of the second halide to be at least about 10 times as high asthat of the first halide in the lowest temperature region of the lamp.Further, the metal contained in the second halide should be less likelyto emit a visible light than the metal contained in the first halide.This implies that, though the metal contained in the second halide mayslightly emit a visible light, the visible light emission from theparticular metal is relatively small.

It should be noted in this connection that the light emitted from Fe orNi contains components having wavelengths of an ultraviolet region in anamount larger than that of the components having wavelengths of avisible region. On the other hand, the light emitted from Ti, Al or Zncontains a large amount of components having wavelengths of a visibleregion. If Ti, Al or Zn is excited singly to emit light, the energy isconcentrated on the particular metal so as to emit a large amount ofvisible light. However, if the metal contained in the second halide hasan energy level higher than that of the metal contained in the firsthalide so as to allow the metal contained in the second halide to beless likely to emit light, the energy is concentrated on the lightemission from the first halide under the condition that the first andsecond halides are present together, with the result that the lightemission from the metal contained in the second halide is suppressed.

The vapor of the second halide functions basically as a buffer gasduring the lighting of the lamp like mercury used in the conventionalmetal halide discharge lamp. Table 1 exemplifies various second halidesused in the present invention and the temperatures at which the vaporpressures of these second halides reach one atmosphere. Incidentally,slight differences are seen depending on the literature in thetemperatures at which the vapor pressures of these second halides reachone atmosphere. However, the values given in Table 1 are considered tobe substantially correct.

TABLE 1 Temperature (° C.) at which Halide Second vapor pressure ofsecond No. Halide halide reaches 1 atm. 1 AlI₃ 422 2 FeI₂ 827 3 ZnI₂ 7274 SbI₃ 427 5 MnI₂ 827 6 CrI₂ 827 7 GaI₃ 349 8 ReI₃ 627 9 MgI₂ 927 10CoI₂ 827 11 NiI₂ 747 12 BeI₂ 487 13 TiI₄ 377 14 ZrI₄ 431 15 HfI₄ 427

Almost all the halides shown in Table 1 have a vapor pressure lower thanthat of mercury. Also, the control range of the lamp voltage is narrowerthan that for mercury. However, the control range of the lamp voltagecan be widened by sealing a plurality of the second halides incombination in the hermetic vessel, as required. For example, where AlI₃is under the state of incomplete evaporation and a desired lamp voltageis not obtained, the lamp voltage remains unchanged even if anadditional AlI₃ is sealed in the hermetic vessel.

On the other hand, if ZnI₂ is sealed in the hermetic vessel in place ofthe additional AlI₃, the lamp voltage can be increased because the lampvoltage produced by the function of ZnI₂ is added to the lamp voltageproduced by the initially sealed AlI₃. Further, if another second halideis added, a higher lamp voltage can be obtained.,

It should also be noted that it is not absolutely necessary for thesecond halide not to emit a visible light. It is acceptable for thesecond halide to emit a visible light, if the ratio of the emittedvisible light to all the visible light emitted from the discharge lampis small enough to make the effect sufficiently small, i.e., the effectgiven by the visible light emitted from the second halide to all thevisible light emitted from the discharge lamp.

Further, a third halide can also be sealed in the hermetic vessel in thepresent invention in addition to the first and second halides in orderto allow the third halide to, for example, correct the arc temperaturedistribution so as to suppress the heat loss.

Halogen . . . It is most desirable in terms of the reactivity to useiodine as a halogen element contained in each of the first and secondhalides. It is also possible to use bromine, chlorine and fluorine as ahalogen element, which exhibit strong reactivity in the order mentioned.In short, any of iodine, bromine, chlorine and fluorine can be used inthe present invention. Further, it is possible to use different halogencompounds in combination. For example, a iodide and a bromide can beused together.

A rare gas, which functions as a starting gas and a buffer gas, is alsosealed in the hermetic vessel in the present invention. Any kind of therare gas can be used in the present invention as far as the rare gasdoes not permeates through the hermetic vessel. It should be noted inthis connection that neon is likely to permeate through a quartz glass.Naturally, where the hermetic vessel is made of a quartz glass, it isdesirable to use argon, krypton or xenon as a rare gas sealed in thehermetic vessel.

If the rare gas is sealed at a high pressure, it is possible to improvethe rising characteristics of the light flux emitted from the metalhalide discharge lamp. Good rising characteristics of the light flux aredesirable for what purpose the discharge lamp may be used, and areparticularly important where the discharge lamp is used in, for example,a headlamp of a vehicle such as an automobile and in a liquid crystalprojector.

Mercury . . . The metal halide discharge lamp of the present inventionpermits “essentially” disusing mercury. In other words, mercury is notsealed at all in the hermetic vessel. However, it is acceptable formercury to be present inside the hermetic vessel in an amount of lessthan 0.3 mg/cc, preferably, not more than 0.2 mg/cc, of the inner volumeof the hermetic vessel. Naturally, in terms of the living environment ofthe human being, it is desirable for mercury not to be sealed at all inthe hermetic vessel of the discharge lamp. Where the electriccharacteristics of the discharge lamp are maintained by the mercuryvapor as in the prior art, mercury is sealed in an amount of at least 20mg/cc of the inner volume of the hermetic vessel in the case of a shortarc type metal halide discharge lamp, and in an amount of at least 5mg/cc of the inner volume of the hermetic vessel in the case of a longarc type metal halide discharge lamp. Compared with the mercury amountrequired in the prior art, it is considered reasonable to state that themetal halide discharge lamp of the present invention permits“essentially” disusing mercury.

Function . . . As described above, the discharge medium used in thepresent invention comprises as a first halide a halide of a metal whichmainly contributes to the emission of a desired light, and as a secondhalide a halide of a metal which is unlikely to emit a visible lightcompared with the metal contained in the first halide. In the presentinvention, the second halide is essentially substituted for mercury usedin the conventional metal halide discharge lamp. As a result, the lampvoltage is determined by mainly the amount of evaporation of the secondhalide in the present invention. Also, the vapor pressure of the halideis determined by the temperature in the coolest region of the hermeticvessel.

The vapor pressure of the second halide during the lighting, which islower than that of mercury but is clearly higher than that of the firsthalide, should be at most about 5 atmospheres.

Under the circumstances, the metal halide discharge lamp of the presentinvention performs a desired function without requiring a substantialmercury sealing, and permits obtaining a lamp voltage sufficient forobtaining the electric characteristics and light-emittingcharacteristics substantially equal to those of the conventional lamprequiring a mercury sealing. Incidentally, the term “substantially”given above denotes that, in the present invention, it is acceptable forthe obtained electrical and light-emitting characteristics to besomewhat inferior to those obtained in the prior art within apractically workable range. The slight difference in question gives riseto no practical problem in view of the fact that it is possible for themetal halide discharge lamp of this type to be lit by an electroniclighting device. However, the lamp voltage can be further increased inthe present invention by applying a heat insulating means to thehermetic vessel, as required.

In the present invention, the hermetic vessel is substantially free fromthe mercury sealing, and the visible light emission is achievedsubstantially solely by the metal contained in the first halide. As aresult, the metal halide discharge lamp of the present invention permitsa good rising of chromaticity in the start-up time. Even where the inputto the lamp is changed, the changes in the color temperature and thecolor rendering properties of the emitted light can be suppressed,making it possible to achieve light control (dimming).

It should also be noted that, in the metal halide discharge lamp of thepresent invention, the lamp characteristics are less affected by theunevenness in the size and shape of the lamps, making it possible tosuppress the unevenness in the color of the emitted light.

Further, an instant re-start up can be achieved easily in the presentinvention because the vapor pressure of the second halide is clearlylower than that of mercury in almost all the cases. It follows that theheight of the starting pulse voltage applied for the re-start up can belowered. As a result, it is possible to lower the dielectric strength ofthe lighting device, start-up device, wiring and illumination apparatus,leading to a low manufacturing cost.

Further, the vapor pressure during the lighting is not extremely high inthe present invention. To be more specific, it is of no difficulty tolower the vapor pressure in question to about 60% of the value requiredin the step of sealing mercury, making it possible to preventsubstantially completely the hermetic vessel from being ruptured duringthe lighting of the lamp.

Still further, the metal halide discharge lamp of the present inventionpermits somewhat improving the color rendering properties on the basisthat the light emitting efficiency is substantially the same.

As described above, each of the short arc type and long arc type metalhalide discharge lamp of the present invention exhibits the steady statecharacteristics substantially equal to those exhibited by the prior art.

In addition, the technical idea of the present invention.can be appliedover a wide range of the lamp power ranging between scores of W andseveral kW.

The metal halide discharge lamp of the present invention may be eitherof a single tube type, in which the hermetic vessel is exposed directlyto the outer atmosphere, or a double wall type, in which the hermeticvessel is inserted into an outer tube. Each of these single tube typeand double wall type lamps of the present invention produces a desiredfunction and effect. Further, if the inner space of the outer tube isheld at a vacuum condition in the double wall type metal halidedischarge lamp, the heat loss can be suppressed so as to further improvethe light emitting efficiency.

What should also be noted is that, where the technical idea of thepresent invention is applied to a short arc type metal halide dischargelamp, it is desirable to construct the lamp to narrow the arc, asdescribed previously. If the arc is narrowed, the light collectingefficiency can be improved. It follows that a marked improvement inbrightness can be obtained, if the short arc type lamp of the particularconstruction is used in, for example, a headlamp of a vehicle such as anautomobile in combination with a reflector, or in an optical system ofthe reflector as in an illumination apparatus for shops or optical fiberillumination apparatus.

According to a second aspect of the present invention, there is provideda metal halide discharge lamp, comprising:

a refractory and transparent hermetic vessel;

a pair of electrodes mounted within the hermetic vessel; and

a discharge medium sealed in the hermetic vessel and containing a firsthalide, a second halide and a rare gas, the first halide being a halideof at least one metal selected from the group consisting of sodium,scandium and a rare earth metal, and the second halide being having arelatively high vapor pressure, and being at least one halide of a metalwhich is unlikely to emit a visible light compared with the metal of thefirst halide.

In the second aspect of the present invention, a visible light isemitted from the first halide. In addition, the metal halides suitablefor the various general uses are specified in the second aspect in viewof the light emitting efficiency and the color rendering properties. Itis possible to use a single compound or a plurality of compounds incombination as the first halide.

The hermetic vessel included in the metal halide discharge lamp of thesecond aspect is essentially free from mercury to enable the presentinvention of the second aspect to produce the function and effectsimilar to those produced by the invention of the first aspect.

According to a third aspect of the present invention, the dischargemedium used in the metal halide discharge lamp of the first or secondaspect of the present invention contain a halide of cesium. Where ahalide of cesium is sealed in the hermetic vessel, the temperaturedistribution of the arc is rendered flat so as to diminish thetemperature gradient, as in the case of sealing mercury, with the resultthat the heat loss in the light emitting tube is decreased. It followsthat the light emitting efficiency is increased to reach a level closeto the case of sealing mercury, compared with the case where a halide ofcesium is not sealed.

To be more specific, cesium generated by decomposition of the cesiumhalide in the arc has a low ionization voltage and, thus, tends to beionized to release an electron even in the intermediate temperatureregion of the arc, which is a relatively low temperature region of thearc. It follows that the presence of cesium in the arc causes theelectron concentration to be increased in the intermediate temperatureregion of the arc.

It should be noted that the electrical conductivity σ is proportional tothe electron density. Since the energy input in a certain temperatureregion is represented by σ E², where E denotes the intensity of theelectric field, the input energy is increased with increase in theelectrical conductivity σ, that is increase in the electron density. Inconclusion, where a halide of cesium is sealed in the hermetic vessel,the energy input is increased in the intermediate temperature region,with the result that the temperature in the intermediate temperatureregion of the arc is elevated.

On the other hand, since the total input to the metal halide dischargelamp is constant, the temperature in the high temperature region of thearc is relatively lowered, as apparent from the energy balance. Itfollows that the temperature distribution of the arc is rendered flat soas to diminish the temperature gradient in the case of sealing a halideof cesium, as in the case of sealing mercury.

On the other hand, in the conventional metal halide discharge lamp inwhich is sealed mercury, mercury also emits light. However, the lightemitting efficiency of mercury itself is not sufficiently high, aspointed out previously. In the present invention, however, the hermeticvessel is essentially free from mercury. Therefore, a metal exhibiting alight emitting efficiency higher than that of mercury, e.g., scandium orsodium, is sealed in the hermetic vessel so as to provide a metal halidedischarge lamp exhibiting a high light emitting efficiency.

Further, disuse of mercury permits producing a function and effectsimilar to those produced by the metal halide discharge lamp accordingto the first aspect of the present invention.

A metal halide discharge lamp according to a fourth aspect of thepresent invention comprises an outer tube housing the light emittingtube and a heat insulating means for suppressing the loss of heatgenerated from the light emitting tube in addition to the metal halidedischarge lamp according to the first aspect of the present invention.Since the loss of heat generated from the light emitting tube issuppressed by the heat insulating means, the light emitting efficiencyof the metal halide discharge lamp is improved. The heat insulatingmeans may be of any construction as far as it is possible to suppressthe loss of heat generated from the light emitting tube. For example,the heat insulating means can be constructed as follows.

Specifically, the inner space of the outer tube is held at a vacuumcondition so as to suppress convection and conduction of the heatgenerated from the light emitting tube. It follows that the heat loss issuppressed so as to keep the discharge medium warm. In this case, theheat insulating means may be of any desired specific construction andshape. Also, any desired material can be used for forming the heatinsulating means. In the present invention, the inner space of the outertube is defined to be held at a vacuum condition. To be more specific,the inner pressure of the outer tube is at most 1.33×10³ Pa.

It is desirable in the metal halide discharge lamp of the presentinvention to use a heat ray reflecting-visible light transmitting filmwhich reflects the heat ray radiated from the light emitting tube to theoutside back into the light emitting tube and which transmits a visiblelight. Use of the particular film permits decreasing the heat lossaccompanying the radiation so as to keep the discharge medium warm. Theheat reflecting-visible light transmitting film can be formed on theinner and/or outer surfaces of a cylindrical body made of quartz glassand interposed between the light emitting tube and the outer tube, onthe inner and/or outer surfaces of the outer tube, and on the outersurface of the light emitting tube.

Needless to say, the particular means described above can be used incombination appropriately.

Since the discharge lamp of the present invention comprises a heatinsulating means for suppressing the loss of heat generated from thelight emitting tube, the loss of heat generated by the discharge withinthe light emitting tube can be suppressed so as to lower the heat lossof the light emitting tube. It follows that the light emittingefficiency is improved.

Further, the hermetic vessel included in the metal halide discharge lampaccording to the fourth aspect of the present invention is substantiallyfree from mercury, making it possible to obtain a function and effectsimilar to those produced by the invention of the first aspect.

According to a fifth aspect of the present invention, there is provideda metal halide discharge lamp which essentially permits disusing mercuryand which is lit by a DC current, comprising;

a refractory and transparent hermetic vessel;

an anode and a cathode fixed to the hermetic vessel; and

a discharge medium sealed in the hermetic vessel and containing a firsthalide, a second halide and a. rare gas, the first halide being a halideof at least one metal selected from the group consisting of sodium,scandium, and a rare earth metal, and the second halide having arelatively high vapor pressure and being a halide of at least one metalwhich is unlikely to emit a visible light compared with the metal of thefirst halide.

If the conventional metal halide discharge lamp having mercury sealedtherein is lit by a DC current, the light emitting metal, e.g., sodiumor scandium, is positively ionized and, thus, sucked toward the cathode,with the result that the concentration of the light emitting metal onthe anode side is rendered lower than that on the cathode side. Also,mercury is somewhat sucked toward the cathode. However, since mercury isoriginally sealed in a predominantly large amount, a sufficiently largeamount of mercury is present on the anode side, too. As a result, thelight emission from the light emitting metal is rendered markedly weakon the anode side to cause light to be emitted from mainly mercury onthe anode side, though a sufficiently large amount of light emissionfrom the light emitting metal can be obtained on the cathode side. Itfollows that a marked color separation is brought about between the twoelectrodes to make the discharge lamp unsuitable for the practical use.Under the circumstances, where the color separation should be avoided, ametal halide discharge lamp requiring the mercury sealing is usedexclusively for the case where the discharge lamp is lit by an ACcurrent.

In the present invention, however, the difference in color temperaturebetween the electrodes is small so as to permit the discharge lamp to beput to practical use in spite of the fact that the second halide issubstantially substituted for mercury, and the discharge lamp is lit bya DC current. It should be noted in this connection that, since thesecond halide is unlikely emit a visible light, the metal contained inthe first halide emits light of high intensity even on the anode side inthe present invention, leading to the small difference in colortemperature between the two electrodes.

It should also be noted that, in a headlamp for a vehicle such as anautomobile or in a lamp for a liquid crystal projector, used is anelectronic lighting device including a metal halide discharge lamp.Where an AC current is used for lighting the discharge lamp, a DCcurrent generated from a battery power source or a DC current obtainedby rectifying an AC current of a commercial frequency is converted ingeneral into a high frequency AC current and, then, supplied to themetal halide discharge lamp.

In the present invention, however, the metal halide discharge lamp isconstructed to be adapted for the lighting with a DC current, making itunnecessary to convert the DC current into a high frequency AC current.This makes it possible to simplify the construction of the electroniccircuit for lighting the discharge lamp so as to make the lightingdevice small in size, light in weight and low in manufacturing cost.

Further, since.mercury is not used, the metal halide discharge lampaccording to the fifth aspect of the present invention produces thefunction and effect similar to those obtained in the invention of thefirst aspect.

According to a sixth aspect of the present invention, there is provideda metal halide discharge lamp which permits essentially disusing mercuryand which is used in a headlamp having a rated power of at most 100 W,comprising:

a refractory and transparent hermetic vessel;

a pair of electrodes fixed to the hermetic vessel; and

a discharge medium sealed in the hermetic vessel and containing a firsthalide, a second halide and a rare gas, the first halide being a halideof at least one metal selected from the group consisting of sodium,scandium, and a rare earth metal, and the second halide having arelatively high vapor pressure and being a halide of at least one metalwhich is unlikely to emit a visible light compared with the metal of thefirst halide.

A metal halide discharge lamp which is used in a headlamp having a ratedpower of at most 100 W is featured in that the distance between the twoelectrodes is small and the tube wall load of the discharge lamp isheavy. Therefore, when it comes to the conventional metal halidedischarge lamp requiring the mercury sealing, a such a high mercuryvapor pressure as at least 20 atmospheres is required in order to obtaina desired lamp voltage, as described previously. As a result, thehermetic vessel tends to be broken relatively easily.

It is also necessary to seal xenon at a high pressure in order toimprove the rising characteristics of the light flux. Specifically, thexenon pressure during the lighting is as high as about 35 atmospheres.It follows that, in order to perform starting by breaking down thestarting gas, it is necessary to apply a starting pulse voltage of ahigh voltage and a large power. Since a higher starting pulse voltage isrequired for the instant start up, it is necessary to increase the gradeof the dielectric strength of the lighting circuit, illuminationapparatus and wiring to conform with the high starting pulse voltage,leading to a high manufacturing cost.

As described above, it is certainly possible to solve the problem inrespect of the rising characteristics of the light flux by sealing xenonat a high pressure, by applying a high starting pulse voltage, and bysupplying a large current immediately after the light, followed bygradually decreasing the current supply. However, the conventional metalhalide discharge lamp remains to be unsatisfactory in the risingcharacteristics of chromaticity. To be more specific, xenon emits lightfirst and, then, mercury emits light. The light emission from mercury iscontinued for 10 to 20 seconds. What should be noted is that the lightemitted from mercury is poor in color rendering properties, failing tofall even within a range of white color.

In the present invention, however, mercury is not sealed in the hermeticvessel, making it possible to reduce the inner pressure of the hermeticvessel to about 60% of the conventional level. It follows that it ispossible to alleviate markedly the problems inherent in the prior art inrespect of the breakage of the hermetic vessel and the starting pulsevoltage.

In the sixth aspect of the present invention, the first halide islimited to a halide of at least one metal selected from the groupconsisting of sodium and scandium. The particular limitation iseffective for obtaining emission of white light required in a headlampat a very high light emitting efficiency.

Further, since mercury is not sealed in the hermetic vessel, a functionand effect similar to those obtained in the first aspect of the presentinvention can also be obtained in the sixth aspect. It follows that themetal halide discharge lamp according to the sixth aspect of the presentinvention is quite suitable for use in a headlamp for a vehicle.

According to a seventh aspect of the present invention, the secondhalide used in the metal halide discharge lamp according to the fifth orsixth aspect of the present invention is limited to a halide of at leastone metal selected from the group consisting of magnesium, iron, cobalt,chromium, zinc, nickel, manganese, aluminum, antimony, beryllium,rhenium, gallium, titanium, zirconium and hafnium. In other words,metals suitable for forming the second halide are specified in theseventh aspect of the present invention.

According to an eighth aspect of the present invention, the secondhalide used in the metal halide discharge lamp according to any of thefirst to third aspects or any of the fifth to seventh aspects of thepresent invention is limited to a halide of at least one metal selectedfrom the group consisting of iron, zinc, manganese, aluminum, andgallium.

In other words, metals most suitable for forming the second halide arespecified in the eighth aspect of the present invention. It should benoted in this connection that the metal halides specified in the eighthaspect are most suitable for use as the main component of the secondhalide. What should be noted is that a further improved lamp voltage canbe obtained, if a halide of at least one metal selected from the groupconsisting of magnesium, cobalt, chromium, nickel, antimony, beryllium,rhenium, titanium, zirconium and hafnium is used as an auxiliarycomponent of the second halide together with the main componentspecified in the eighth aspect.

According to a ninth aspect of the present invention, the second halideused in the metal halide discharge lamp according to any of the first tothird aspects or any of the fifth to eighth aspects of the presentinvention is sealed in the hermetic vessel in an amount of 0.05 to 200mg/cc of the inner volume of the hermetic vessel.

In the ninth aspect of the present invention, specified is a generalrange of the sealing amount of the second halide. Depending on thespecific halide selected as the second halide, a suitable range of thesealing amount is narrower than noted above. However, the range of thesealing amount noted above should be taken as a generally satisfactoryrange.

In a tenth aspect of the present invention, the second halide used inthe metal halide discharge lamp according to the sixth aspect of theinvention is defined to be sealed in the hermetic vessel in an amount of1 to 200 mg/cc of the inner volume of the hermetic vessel. Defined inthe tenth aspect is a suitable sealing amount of the second halide,covering the case where the metal halide discharge lamp is used in aheadlamp for a vehicle.

According to an eleventh aspect of the present invention, the rare gassealed in the hermetic vessel of the metal halide discharge lampaccording to the first to tenth aspects of the present invention isdefined to be sealed at a pressure of at least one atmosphere. Theeleventh aspect is to increase the pressure of the rare gas sealed inthe hermetic vessel so as to improve the rising characteristics of thelight flux. The good rising characteristics of the light flux, which aredesirable for any use of the discharge lamp, are particularly desirablewhere the discharge lamp is used in a headlamp of a vehicle. It isdesirable to use a small short arc type metal halide discharge lampwhere the discharge lamp is used in a headlamp for a vehicle.

In a twelfth aspect of the present invention, the rare gas sealed in thehermetic vessel of the metal halide discharge lamp according to thesixth to tenth aspects of the present invention is defined to be sealedat a pressure of 1 to 15 atmospheres. Defined in the twelfth aspect is arare gas sealing pressure suitable for the case where the metal halidedischarge lamp is used in a headlamp of a vehicle.

According to a thirteenth aspect of the present invention, the hermeticvessel included in the metal halide discharge lamp according to thesixth, tenth or twelfth aspects of the present invention is defined tohave an inner diameter of 3 to 10 mm and an outer diameter of 5 to 13mm. Defined in the thirteenth aspect is the size of the hermetic vesselsuitable for the case where the metal halide discharge lamp is used in aheadlamp of a vehicle.

According to a thirteenth aspect of the present invention, the hermeticvessel included in the metal halide discharge lamp according to thesixth, tenth or twelfth aspects of the present invention is defined tohave an inner diameter of 3 to 10 mm and an outer diameter of 5 to 13mm. Defined in the thirteenth aspect is the size of the hermetic vesselsuitable for the case where the metal halide discharge lamp is used in aheadlamp of a vehicle.

According to a fourteenth aspect of the present invention, the distancebetween the two electrodes mounted in the hermetic vessel included inthe metal halide discharge lamp according to the sixth, tenth, twelfthand thirteenth aspects of the present invention is defined to be 1 to 6mm. Defined in the fourteenth aspect is the distance between the twoelectrodes mounted in the hermetic vessel, the distance being suitablefor the case where the metal halide discharge lamp is used in a headlampof a vehicle. If the distance between the electrodes exceeds 6 mm, thedischarge lamp fails to act as a dot light source, leading to a lowlight collecting effect. More preferably, the distance between theelectrodes should fall within a range of between 1 and 5 mm.

According to a fifteenth aspect of the present invention, the metalhalide discharge lamp according to the sixth, tenth, and twelfth tofourteenth aspects of the present invention is defined to be constructedso as to be lit by a DC current. Defined in the fifteenth aspect is thedischarge lamp is lit by a DC current so as to miniaturize the lightingdevice when the discharge lamp is used in a headlamp for a vehicle andto lower the manufacturing cost of the discharge lamp. To be morespecific, a battery power source is generally mounted to a vehicle suchas an automobile. Therefore, use of a lighting device utilizing a DCcurrent permits simplifying the circuit construction, compared with thecase where a DC current is converted first into an AC current and, then,supplied to the metal halide discharge lamp so as to light the dischargelamp. The particular effect remains unchanged even in the case where acontrol means such as a voltage increasing chopper or a voltagedecreasing chopper is used for controlling the DC power to have adesired voltage, because the particular control means is used, whenrequired, even in the case of using an AC power for lighting thedischarge lamp. In the present invention, mercury is not sealed in thehermetic vessel, with the result that the color separation problem neednot be worried about in practice. It follows that it is possible to usea DC current for lighting the discharge lamp of the present invention.

According to a sixteenth aspect of the present invention, the dischargemedium used in the metal halide discharge lamp according to the sixth,tenth, and twelfth to fifteenth aspects of the present invention isdefined to contain a halide of cesium. In the sixteenth aspect of thepresent invention, a halide of cesium is sealed in the hermetic vesselincluded in the metal halide discharge lamp used in a headlamp for avehicle so as to flatten the arc gradient and, thus, to improve thelight emitting efficiency. As a matter of fact, the light emittingefficiency of the discharge lamp according to the sixteenth aspect ofthe present invention is higher than that of the conventional metalhalide discharge lamp having mercury sealed therein.

According to a seventeenth aspect of the present invention, the metalhalide discharge lamp according to the sixth, tenth and twelfth tosixteenth aspects of the present invention is defined to furthercomprise an outer tube having the hermetic vessel housed therein andhaving the inner space kept at a vacuum condition. In the seventeenthaspect of the present invention, the hermetic vessel included in themetal halide discharge lamp used in a headlamp of a vehicle is housed inthe outer tube having the inner space held at a vacuum condition, withthe result that the light emitting efficiency of the discharge lamp isrendered higher than that of the conventional discharge lamp havingmercury sealed therein.

According to an eighteenth aspect of the present invention, the metalhalide discharge lamp according to the sixth, tenth, and twelfth toseventeenth aspects of the present invention further comprises means forremoving ultraviolet light which permits “substantially” removing anultraviolet light from the light led to the outside. The expression“substantial” removal denotes that the ultraviolet light is removed to apractically allowable level. In other words, the substantial removaldoes not necessarily imply that 100% of the ultraviolet light isremoved.

The ultraviolet light removing means may be of any construction as faras the ultraviolet light is substantially removed. For example, thelight emitting tube is housed in an outer tube made of a glass materialof the composition capable of removing the ultraviolet light.Incidentally, it is possible for the outer tube to communicate with theouter atmosphere. Alternatively, the outer tube may be hermetic and mayhave the inner space held at a vacuum condition.

It is also possible to impart an ultraviolet light removing function tothe inner surface of the light emitting tube or to the light emittingtube itself. To be more specific, the ultraviolet light shieldingfunction can be imparted by converting the material texture of the inneror outer surface of the light emitting tube into an ultraviolet lightshielding texture or by forming a transparent film capable of shieldingan ultraviolet light on the inner or outer surface of the light emittingtube. Further, a pair of ultraviolet light shielding cylinders may bearranged outside the light emitting tube.

Since the ultraviolet light led to the outside is substantially removedin the present invention, the headlamp is prevented from beingdeteriorated by the ultraviolet light. Also, the human eyes areprevented from being irradiated with an ultraviolet light. Further, inthe case of using an outer tube, the hermetic vessel is mechanicallyprotected by the outer tube.

According to an additional aspect of the present invention, there isprovided a lighting device for a metal halide discharge lamp,comprising:

a metal halide discharge lamp defined in any one of the sixth, tenth andtwelfth to eighteenth aspects of the present invention; and

a lighting circuit constructed to supply current in an amount at leastthree times as much as a rated lamp current immediately after thelighting of the metal halide discharge lamp, followed by decreasing thecurrent with time.

Defined in this additional aspect of the present invention is a lightingdevice for a metal halide discharge lamp which meets the risingcharacteristics of the light flux required for a headlamp for a vehicle.The lighting circuit may be operated by either an AC current or a DCcurrent. Also, the lighting circuit may be of any desired constructionas far as the requirements given above are satisfied.

Further, according to still additional aspect of the present invention,there is provided an illumination apparatus, comprising:

an illumination apparatus body; and

a metal halide discharge lamp defined in any one of the first toeighteen aspects of the present invention, the metal halide dischargelamp being supported by the illumination apparatus body.

The invention of the still additional aspect noted above is applicableto any of the apparatuses in which the metal halide discharge lampaccording to any of the first to eighteenth aspects of the presentinvention is used for the illumination purpose. When it comes to a shortarc type metal halide discharge lamp, the illumination apparatus of thepresent invention is suitable for use in illumination apparatus usingthe discharge lamp in combination with an optical system such as areflector or a lens, e.g., a liquid crystal projector or an overheadprojector, in a headlamp for a vehicle such as an automobile, and inillumination apparatus for shops such as an optical fiber illuminationapparatus and a spot light.

On the other hand, when it comes to a long arc type metal halidedischarge lamp, the discharge lamp of the present invention can besuitably used in various illumination apparatuses for the generalillumination purposes such as a down light, an illuminating lamp mounteddirectly to the ceiling, an illumination apparatus for roads, anillumination apparatus for tunnels, in light projectors, and in adisplay apparatus.

To reiterate, the metal halide discharge lamp of the present inventionis featured in that a halide of a metal which is unlikely to emit avisible light compared with a light emitting metal is sealed in thehermetic vessel in place of mercury. The particular metal halide issealed in a high vapor pressure together with a halide of the lightemitting metal. The particular construction defined in the presentinvention makes it possible to provide a metal halide discharge lampwhich produces electric characteristics and light emittingcharacteristics substantially equal to those produced by theconventional metal halide discharge lamp having mercury sealed therein.

The present invention also provides a metal halide discharge lamp whichproduces at least one of the auxiliary effects a) to e) given below:

a) Good rising characteristics of spectral characteristics at thestart-up time.

b) Capability of light control (dimming).

c) Small unevenness in characteristics.

d) Easy instant re-start up.

e) High resistance to rupture of hermetic vessel.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinbefore.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a schematical view illustrating the lamp voltage in a metalhalide discharge lamp;

FIG. 2 is a graph showing an emission spectrum distribution of aconventional short arc type metal halide discharge lamp for lightprojection;

FIG. 3 is a front view showing a metal halide discharge lamp accordingto a first embodiment of the present invention;

FIG. 4 is a schematical view showing the construction of an opticalsystem of an RGB color separation type liquid crystal light projector;

FIG. 5 is a graph showing the arc temperature distribution for lamp 2(present invention) and lamp 1 (prior art), which are shown in Table 1;

FIG. 6 is a cross sectional view showing a part of a lamp for a liquidcrystal projector, said lamp comprising the metal halide discharge lampaccording to the first embodiment of the present invention and areflector formed integral with the discharge lamp;

FIG. 7 is a schematical view showing a liquid crystal projector usingthe lamp for the projector shown in FIG. 6 as an illuminating apparatusaccording to the first embodiment of the present invention;

FIG. 8 is a cross sectional view showing a metal halide discharge lamaccording to a second embodiment of the present invention;

FIG. 9 is a graph showing the relationship between the xenon sealingpressure and the rising time of the light flux in the metal halidedischarge lamp according to the second embodiment of the presentinvention;

FIG. 10 is a graph showing the relationship between the sealing amountof FeI₂ used as a second halide and the lamp voltage in the metal halidedischarge lamp according to the second embodiment of the presentinvention;

FIG. 11 is a front view showing a metal halide discharge lamp accordingto a third embodiment of the present invention;

FIG. 12 is an oblique view showing a headlamp used in a vehicle such asan automobile as an illumination device according to the secondembodiment of the present invention;

FIG. 13 is a front view showing a metal halide discharge lamp accordingto a fourth embodiment of the present invention;

FIG. 14 is a graph showing the spectral distribution of a conventionallong arc type metal halide discharge lamp;

FIG. 15 is a graph showing the spectral distribution of a long arc typemetal halide discharge lamp according to the fourth embodiment of thepresent invention;

FIG. 16 is a front view showing a metal halide discharge lamp accordingto a fifth embodiment of the present invention;

FIG. 17 is a chromaticity diagram showing the rising in the spectralcharacteristics of the metal halide discharge lamp according to thefifth embodiment of the present invention in comparison with the priorart;

FIG. 18 is a front view showing the gist portion of a metal halidedischarge lamp according to a sixth embodiment of the present invention;

FIG. 19 is a front view showing a metal halide discharge lamp accordingto a seventh embodiment of the present invention;

FIG. 20 is a chromaticity diagram showing the changes in chromaticity ofthe discharge lamp 2 (present invention) and the conventional dischargelamp 1 shown in Table 22 in Embodiment 17 described herein later;

FIG. 21 is a front view showing a metal halide discharge lamp accordingto an eighth embodiment of the present invention;

FIG. 22 is a chromaticity diagram showing the rising characteristics inthe spectral characteristics of the discharge lamp according to theeighth embodiment of the present invention in comparison with the priorart;

FIG. 23 is a graph showing the relationship between the rare gas sealingpressure and the rising time of the light flux, which covers the metalhalide discharge lamp according to the eight embodiment of the presentinvention shown in FIG. 21;

FIG. 24 is a graph showing the relationship between the sealing amount(mg/cc) of ZnI₂ used as the second halide and the lamp voltage (V),which covers the metal halide discharge lamp according to the eightembodiment of the present invention shown in FIG. 21;

FIG. 25 is a circuit diagram showing the lighting device of a metalhalide discharge lamp according to the first embodiment of the presentinvention;

FIG. 26 is a circuit diagram showing the lighting device of a metalhalide discharge lamp according to the second embodiment of the presentinvention;

FIG. 27 is a schematical view showing a headlamp for a vehicle as athird embodiment of the illumination device of the present invention;

FIG. 28 is a schematical view showing a light distributor portionincluded in the headlamp for a vehicle as the third embodiment of theillumination device of the present invention; and

FIG. 29 is a schematical view showing a down light as a fourthembodiment of the illumination apparatus of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Let us describe some embodiments of the present invention with referenceto the accompanying drawings.

Embodiment 1:

FIG. 3 is a front view showing a metal halide discharge lamp accordingto the first embodiment of the present invention. As shown in thedrawing, the discharge lamp comprises a hermetic vessel 1, a pair ofelectrodes 2, metal foils 3 for the sealing, and outer lead wires 4. Themetal halide discharge lamp of this embodiment is of a short arc type.

The hermetic vessel 1 is prepared by rotating under heat a quartz glasstube having an inner diameter of 14 mm into a bulb having an ellipticalcross section. A pair of slender sealing portions 1 a, 1 a areintegrally fixed to the ends in the long axis direction of theelliptical hermetic vessel 1.

The electrode 2 comprises an electrode shaft 2 a and an electrode coil 2b. The tip portion of the electrode shaft 2 a somewhat projects inwardand, the electrode coil 2 b is wound about the projecting tip portion ofthe electrode shaft 2 a. The proximal end portion of the electrode shaft2 a is welded to one end of the sealed metal foil 3 within the sealingportion 1 a. In this embodiment, the distance between the tips of theelectrodes 2 is set at 4 mm. The sealed metal foil 3 consists of amolybdenum foil, which is hermetically sealed within the sealing portion1 a, and the outer lead wire 4 is welded to the other end of the metal(molybdenum) foil 3.

Sealed in the hermetic vessel 1 is a discharge medium consisting of arare gas, a first halide, and a second halide. To be more specific, anargon gas was sealed as the rare gas at a pressure of 6.65×10⁴ Pa.Dysprosium iodide (DyI₃) was sealed as the first halide in an amount of1 mg together with 1 mg of neodymium iodide (NdI₃). Further, 8 mg ofeach of the halides shown in Table 2 was sealed as the second halide.

The lamp voltage, light emitting efficiency and color temperature weremeasured with the input power set constant at 150 W in respect of theresultant short arc type metal halide discharge lamps, with the resultsas shown in Table 2. The data on the prior art, which was equal to themetal halide discharge lamp of the present invention except that 13 mgof mercury was sealed in place of the second halide, are also shown inTable 2.

TABLE 2 Light Screen Second Lamp emitting Color brightness Lamp No.halide voltage efficiency temp. ratio 1* — 75 V 71 lm/W 8700K 1.0 2 AlI₃62 V 72 lm/W 9120K 1.4 3 FeI₂ 70 V 70 lm/W 9210K 1.35 4 ZnI₂ 73 V 68lm/W 9160K 1.42 5 SbI₃ 63 V 73 lm/W 8930K 1.35 6 MnI₂ 55 V 72 lm/W 9040K1.42 7 CrI₂ 58 V 69 lm/W 9100K 1.45 8 GaI₃ 59 V 68 lm/W 9030K 1.39 9ReI₃ 61 V 70 lm/W 9240K 1.37 10 TiI₄ 72 V 70 lm/W 9220K 1.38 Note:*Prior Art

As apparent from Table 2, any of the metal halide discharge lamps of thepresent invention exhibited a lamp voltage higher than 50V, and wasfound to be substantially equal to the conventional metal halidedischarge lamp (sample No. 1) in the light emitting efficiency and thecolor temperature.

Then, the screen brightness ratio was measured by using each of themetal halide discharge lamps shown in Table 2 in combination with theoptical system of an RGB color separation type liquid crystal projectorshown in FIG. 4, with the results as shown in Table 2. As shown in FIG.4, the optical system comprises a metal halide discharge lamp 5 equal tothat shown in FIG. 3, a reflector 6, an ultraviolet light-infrared lightcut filter 71, color separation dichroic mirrors 8 a, 8 b, liquidcrystal panels 9 _(B), 9 _(G), 9 _(R), mirrors 10 a, 10 b, colorsynthesis mirrors 11 a, 11 b, and a projecting lens 12. The capitalletters B, G, R shown in FIG. 4 represent the blue optical axis, greenoptical axis and red optical axis, respectively. The liquid crystalpanels 9 _(B), 9 _(G), and 9 _(R) are driven by image signals of blue,green and red, respectively. Table 2 shows that the screen brightnessfor the metal halide discharge lamp of the present invention was about1.4 times as high as that for the prior art.

Further, the arc temperature distribution was measured for each of thedischarge lamp of the present invention (sample No. 2) and theconventional discharge lamp (sample No. 1), with the results as shown inFIG. 5. To be more specific, FIG. 5 is a graph showing the arctemperature distribution for samples No. 1 (prior art) and No. 2(present invention). Plotted on the abscissa of the graph is theposition in the radial direction of the hermetic vessel in a crosssection perpendicular to the axis of the hermetic vessel and passingthrough the center between the two electrodes. On the other hand,plotted on the ordinate of the graph is the arc temperature (absolutetemperature K). Curve A shown in FIG. 5 denotes the discharge lamp ofthe present invention (sample No. 2), with curve B representing theconventional discharge lamp (sample No. 1). As apparent from the graph,the arc is rendered narrower in the discharge lamp of the presentinvention (curve A).

Further, the color temperatures of samples Nos. 2 and 3 (presentinvention) and sample No. 1 (prior art) of the discharge lamps weremeasured, covering the cases where these discharge lamps were lit underinput powers of 70 W, 90 W, 110 W and 130 W, with the results as shownin Table 3:

TABLE 3 Sample No. 70W 90W 110W 130W 1 6510K 6930K 7560K 8030K (priorart) 2 8630K 8740K 8900K 9060K 3 8720K 8860K 9030K 9180K

As described previously, where the input power is lowered, the lightemission from mercury is rendered relatively predominant in thedischarge lamp of the prior art (sample No. 1), leading to a markedreduction in the color temperature.

In the discharge lamps of the present invention (samples Nos. 2 and 3),however, the hermetic vessel is substantially free from mercury. Also,the emission of a visible light caused by the second halide is small. Itfollows that light is emitted mainly from the light emitting metalcontained in the first halide even in the case of lowering the inputpower. It should also be noted that the color temperature is somewhatlowered because the vapor pressure of the light emitting metal islowered with decrease in the input power.

In the experiments described above, the input power was decreased from150 W (see Table 2) to 70 W (see Table 3). In this case, the colortemperature was changed by 2190K; whereas, the change in the colortemperature was at most 500K in the discharge lamps of the presentinvention.

Further, re-starting was evaluated, with the results as shown in Table4:

TABLE 4 Sample No.  1* 2 3 4 5 6 7 8 9 10 Re-start 12 4 3 5 3 4 4 5 3  6voltage kV Note: *Prior Art

As apparent from Table 4, the re-starting voltage is low in thedischarge lamp of the present invention. This is because the vaporpressure of the second halide during the lighting is lower than that ofmercury. To be more specific, the vapor pressure of the second halidein, for example, sample No. 3 of the discharge lamp of the presentinvention is 0.6 atmosphere, and the vapor pressures of the other secondhalides are at most 5 atmospheres. In sample No. 1 of the prior art,however, the vapor pressure of mercury is as high as 28 atmospheres,leading to the high re-start up voltage as shown in Table 4.

FIG. 6 is a front view partly including a cross sectional view, showinga discharge lamp for a projector comprising the metal halide dischargelamp according to the first embodiment of the present invention and areflector formed integral with the discharge lamp. The referencenumerals common with FIGS. 3 and 6 denote the same members and, thus,reference thereto is omitted in the following description. In theembodiment shown in FIG. 6, a metal halide discharge lamp 5 equal to thedischarge lamp shown in FIG. 3 and a reflector 6 are made integral. Areference numeral 5 b shown in FIG. 6 represents a heat insulating filmformed to cover the outer surface of the hermetic vessel 1 of the metalhalide discharge lamp 5, said hermetic vessel 1 surrounding theelectrode on the side of the light-projecting opening of the reflector6.

The reflector 6, which is formed of a curved glass plate having aparabolic cross section, comprises a reflector main body 6 b and a neckportion 6 a positioned at the tip of the parabolic configuration. Amulti-layered interference reflecting film 6 c, which reflects a visiblelight and transmits an infrared light, is formed on the inner surface ofthe reflector main body 6 b. Further, a through-hole 6 d is formed inthe reflector main body 6 b.

The metal halide discharge lamp 5 includes a base 5 a, which is fitwithin the neck portion 6 a of the reflector 6 and fixed by a basecement 7. Further, a power supply wire 8 is led to the back side of thereflector 6 via the through-hole 6 d of the reflector 6.

A reference numeral 9 represents an electronic lighting device servingto supply an electric power of a desired voltage and a desired lampcurrent to the metal halide discharge lamp 5.

Embodiment 2:

FIG. 7 schematically shows a liquid crystal projector as a firstembodiment of the illumination apparatus of the present invention. Thedischarge lamp for the projector shown in FIG. 6 is used in the liquidcrystal projector shown in FIG. 7. The reference numerals common withFIGS. 6 and 7 represent the same members of the projector and, thus,reference thereto is omitted in the following description.

As shown in the drawing, the liquid crystal projector comprises a liquidcrystal display means 11, an image control means 12, an optical system13, a body case 14, and a screen 15. The liquid crystal display means 11serves to display the image to be displayed by utilizing a liquidcrystal material. To be more specific, the liquid crystal display means11 is irradiated with light emitted from behind the display means 11 bythe metal halide discharge lamp 5 included in the metal halide dischargelamp apparatus and collected by the reflector 6. The image control means12 serves to drive and control the liquid crystal display means 11. Inshort, the image control means 12 also performs the function of atelevision receiver. The liquid crystal display means 11 and the imagecontrol means 12 are housed in the body case 14. Further, the opticalsystem 13 serves to project the light passing through the liquid crystaldisplay means 11 onto the screen 15.

Embodiment 3:

FIG. 8 is a cross sectional view, or front view, showing a metal halidedischarge lamp according to a second embodiment of the presentinvention. The reference numerals common with FIGS. 3 and 8 denote thesame members of the discharge lamp and, thus, reference thereto isomitted in the following description.

The metal halide discharge lamp of the second embodiment is also of ashort arc type, and differs from the discharge lamp of the firstembodiment in that, in the second embodiment, the inner volume of thehermetic vessel 1 is as small as 0.05 cc. It should also be noted that,in the second embodiment, the hermetic vessel 1 has an inner diameter of4 mm. An electrode coil is not wound about the electrode 2. Further, thedistance between the two electrodes is 4.2 mm.

The discharge medium used in the second embodiment consisted of a xenongas sealed at 1 atmosphere, a first halide including scandium iodide(ScI₃) sealed in an amount of 0.14 mg and sodium iodide (NaI) sealed inan amount of 0.86 mg, and a second halide shown in Table 5. The secondhalide was sealed in the hermetic vessel 1 in an amount of 1 mg. Themetal halide discharge lamps thus prepared were tested for the lampvoltage, light emitting efficiency, general color rendering index,hereinafter referred to as “color rendering properties Ra”, and colortemperature, with the power input set constant at 35 W. Table 5 showsthe results together with the results for sample No. 1 for theconventional metal halide discharge lamp. The conventional dischargelamp (sample No. 1) was equal to the discharge lamps of the presentinvention, except that 1 mg of mercury was sealed in sample No. 1 inplace of the second halide sealed in the discharge lamps of the presentinvention.

TABLE 5 Color Light rendering Second Lamp emitting properties ColorSample No. halide voltage efficiency (Ra) temperature 1 — 83 V 80 lm/W63 4120K (prior art) 2 AlI₃ 62 V 78 lm/W 65 3860K 3 FeI₂ 70 V 73 lm/W 714210K 4 ZnI₂ 75 V 78 lm/W 65 3830K 5 SbI₃ 63 V 75 lm/W 66 3790K 6 MnI₂55 V 72 lm/W 68 3950K 7 CrI₂ 58 V 74 lm/W 65 3860K 8 GaI₃ 59 V 76 lm/W66 3760K 9 ReI₃ 61 V 78 lm/W 64 3840K

As apparent from Table 5, the lamp voltage for the metal halidedischarge lamp of the present invention was found to be higher than 50V.Also, the discharge lamp of the present invention, which was somewhatlower in the light emitting efficiency than the prior art (sample No.1), was found to permit improving the color rendering properties. Theexperimental data given in Table 5 clearly supports that the metalhalide discharge lamp of the present invention is substantially equal tothe prior art in the characteristics under the stationary condition.

Then, sample No. 3 of the discharge lamp of the present invention andsample No. 1 of the conventional discharge lamp were tested for thecolor rendering properties and the color temperature under the conditionthat each of these discharge lamps was lit with an input power of 15 W,20 W, 25 W and 30 W, with the results as shown in Table 6:

TABLE 6 Sample No. 15W 20W 25W 30W 1 (Prior Art) Color Rendering 40 4558 61 Properties (Ra) Color Temperature (K) 5640 4970 4630 4350 3(Present Invention) Color Rendering 63 64 66 69 Properties (Ra) ColorTemperature (K) 4530 4440 4310 4240

As shown in Table 6, the changes in the color temperature and colorrendering properties were 1520K and 23, respectively, in theconventional discharge lamp (sample No. 1), where the input power waschanged from 35 W (see Table 5) to 15 W. These changes are very large,making it practically impossible to apply light control (dimming) to thedischarge lamp.

In the discharge lamp of the present invention (sample No. 3), however,the changes in the color temperature and color rendering properties wereas small as 320K and 8, respectively, with the result that it is of nodifficulty to apply light control to the discharge lamp.

Further, re-starting of the discharge lamps (samples Nos. 1 and 3) wasalso evaluated. In this experiment, sample No. 10 of a discharge lamp,which was equal to the discharge lamp of sample No. 3 except that axenon gas was sealed in the hermetic vessel at a pressure of 1.33×10⁴Pa, was also tested for the re-starting voltage. Table 7 shows theresults:

TABLE 7 Sample No. 1 prior art 3 10 Re-starting 14 7 3 voltage (kV)

Table 7 clearly shows that the re-starting voltage for the dischargelamp of the present invention is substantially less than half the valuefor the prior art (sample No. 1). Particularly, a marked improvement wasrecognized in sample No. 10 in which the rare gas (xenon) was sealed ata low pressure without placing a high importance to the rising of thelight flux.

FIG. 9 is a graph showing the relationship between the sealing pressureof xenon gas and the rising time of the light flux in the metal halidedischarge lamp according to the second embodiment of the presentinvention. In this graph, the xenon sealing pressure (atmospheres) isplotted on the abscissa, with the rising time (seconds) of the lightflux being plotted on the ordinate. As apparent from FIG. 9, the risingtime of the light flux is markedly shortened where the xenon sealingpressure is higher than one atmosphere, but is prominently long wherethe xenon sealing pressure is lower than one atmosphere.

FIG. 10 is a graph showing the relationship between the sealing amountof iron iodide (FeI₂) as the second halide and the lamp voltage inrespect of the metal halide discharge lamp according to the secondembodiment of the present invention. In this graph, the sealing amountof iron iodide (mg/cc) is plotted on the abscissa, with the lamp voltage(V) being plotted on the ordinate. FIG. 10 shows that the lamp voltageexceeds 30V where the FeI₂ sealing amount exceeds 1 mg/cc of the innervolume of the hermetic vessel. Incidentally, where the FeI₂ sealingamount exceeds 200 mg/cc of the inner volume of the hermetic vessel,FeI₂ partly fails to be evaporated. The FeI₂ failing to be evaporatedabsorbs light, leading to a low light emitting efficiency of thedischarge lamp.

Embodiment 4:

FIG. 11 is a front view showing a metal halide discharge lamp accordingto a third embodiment of the present invention. In this embodiment, asmall short arc type metal halide discharge lamp similar to that shownin FIG. 8 is constructed to be adapted for mounting in a headlamp for avehicle such as an automobile. As seen from the drawing, the dischargelamp of the third embodiment comprises an outer tube 21, a base 22 andan insulating tube 23.

The outer tube 21 is capable of shielding an ultraviolet light. A metalhalide discharge lamp 5′ substantially equal in construction to thedischarge lamp shown in FIG. 8 is housed in the outer tube 21. Also, theouter tube 21, which is fixed at both ends to the sealing portions 1 a,is not hermetic but communicates with the outer atmosphere. One sealingportion 1 a is mounted to the base 22. The outer lead wire 4 led out ofthe other end extends in parallel to the outer tube 21 so as to beintroduced into the base 22 and connected to a terminal (not shown)within the base 22. Further, the outer lead wire 4 is covered with theinsulating tube 23.

Embodiment 5:

FIG. 12 is an oblique view showing a headlamp for a vehicle such as anautomobile as a second embodiment of the illumination apparatus of thepresent invention. As shown in the drawing, the headlamp of the presentinvention comprises a reflector 31 and a front cover 32. The reflector31 is formed of a curved plastic plate having a parabolic cross sectionincluding a tip on the back surface. A metal halide discharge lamp shownin FIG. 11, which is not shown in FIG. 12, is attached to or detachedfrom the reflector 31 from the back surface at the tip of the reflector31.

On the other hand, the front cover 32, which is formed of a transparentplastic plate, is mounted to close hermetically the front opening of thereflector 31. Incidentally, a prism or a lens is formed integral withthe front cover 32.

Embodiment 6:

FIG. 13 is a front view showing a metal halide discharge lamp accordingto a fourth embodiment of the present invention. As shown in thedrawing, the discharge lamp of the fourth embodiment comprises a lightemitting tube 41, a first support band 42, a first conductor frame 43, aflare stem 44, a bimetal and starting resistor 45, a second support band46, a second conductor frame 47, a conductor wire 48, an outer tube 49and a base 50.

The light emitting tube 41 comprises a slender quartz glass tube havingan inner diameter of 20 mm. A pair of main electrodes are sealed at bothends of the quartz glass tube. Also, a starting auxiliary electrode issealed in the vicinity of one of the main electrodes. The distancebetween the main electrodes is set at 42 mm.

The first support band 42 is fixed to the first conductor frame 43 in amanner to embrace a pinch sealing portion above the light emitting tube41 in the drawing. The first conductor frame 43, which is fixed to theflare stem 44, serves to apply a voltage to the main electrode above thelight emitting tube 41. The flare stem 44 is fixed to a neck portion ofthe outer tube 49. Further, the bimetal or starting resistor 45 forms astarting circuit and serves to apply at the starting time a voltage of apolarity opposite to that of the main electrode formed in the vicinityof the starting auxiliary electrode.

The second support band 46 is fixed to the second conductor frame 47 ina manner to embrace the pinch sealing portion in a lower portion of thelight emitting tube 41 in the drawing. The second conductor frame 47 isfixed to a top portion of the outer tube 49. Further, the conductor wire48 is connected at one end to a conductor wire of the flare stem 44 andat the other end to the second support band 46 so as to be connected tothe other main electrode of the light emitting tube 41 via the secondconductor frame 47.

The light emitting tube 41 of the construction described above and thebimetal or starting resistor 45 are mounted within the outer tube 49.Further, an initial getter (not shown) is mounted within the outer tube49 so as to have the impurity gases within the outer tube 49 adsorbed onthe getter.

Sealed inside the light emitting tube 41 were 3 mg of scandium iodide(ScI₃) and 15 mg of sodium iodide (NaI) as the first halides. Alsosealed was 20 mg of the second halide shown in Table 8 together with2.66×10³ pa of argon as a rare gas. As a result, prepared were 15 kindsof metal halide discharge lamps. Among these discharge lamps, sample No.14 of the discharge lamp was equal to sample No. 2 of the dischargelamp, except that 5 mg of ZnI₂ was further sealed in the light emittingtube in sample No. 14. Likewise, sample No. 15 of the discharge lamp wasequal to sample No. 10 of the discharge lamp, except that 5 mg of FeI₂was further sealed in the light emitting tube in sample No. 15. Each ofthese samples 14 and 15 was intended to increase the lamp voltage bysealing a plurality of the second halide compounds in the light emittingtube.

For comparison, a conventional metal halide discharge lamp was alsoprepared as sample No. 1 by sealing 40 mg of mercury in the lightemitting tube in place of the second halide specified in the presentinvention. These discharge lamps including the conventional dischargelamp (sample No. 1) were lit with the lamp input power set constant at400 W for evaluating the lamp voltage, light emitting efficiency, colortemperature and color rendering properties. The results are shown inTable 8:

TABLE 8 Color Light rendering Sample Second Lamp emitting propertiesColor No. halide voltage efficiency (Ra) temperature 1 — 132 V 101 lm/W62 4320K (Prior Art) 2 AlI₃ 112 V 96 lm/W 65 4120K 3 FeI₂ 118 V 95 lm/W68 4510K 4 ZnI₂ 120 V 98 lm/W 65 4160K 5 SbI₃ 114 V 94 lm/W 69 4040K 6MnI₂ 83 V 93 lm/W 64 4210K 7 CrI₂ 109 V 96 lm/W 68 4260K 8 GaI₃ 125 V 97lm/W 67 4130K 9 ReI₃ 103 V 91 lm/W 69 4240K 10 MgI₂ 78 V 95 lm/W 664140K 11 CoI₂ 118 V 95 lm/W 68 4480K 12 NiI₂ 109 V 95 lm/W 69 4410K 13BeI₂ 95 V 93 lm/W 63 4210K 14 AlI₃ + 137 V 97 lm/W 65 4150K ZnI₂ 15MgI₂ + 105 V 95 lm/W 67 4210K FeI₂

Let us describe the electrical characteristics of the metal halidedischarge lamp according to the fourth embodiment of the presentinvention.

As apparent from Table 8, the lamp voltage of the conventional metalhalide discharge lamp is determined by the sealing amount of mercury. Onthe other hand, the lamp voltage of the discharge lamp according to thefourth embodiment of the present invention is determined mainly by theamount of evaporation of the second halide. In this case, if a heatinsulating means is mounted to the light emitting tube, it is possibleto permit, for example, FeI₂ sealed in sample No. 3 of the dischargelamp to be evaporated in an amount required for obtaining a desired lampvoltage. It follows that it is possible to obtain a lamp voltage fullycomparable with that of the conventional discharge lamp by applying aheat insulating means to the light emitting tube 41.

Then, let us describe the light emitting characteristics. Specifically,in sample No. 3 of the discharge lamp, a visible light is emittedslightly from iron contained in the second halide of FeI₂. However,light emission from mercury is not observed. In sample No. 3, the lightemitting efficiency is somewhat lowered. However, the color renderingproperties are somewhat improved. It should be noted that, a strongultraviolet light is emitted if an iron halide is sealed singly in thelight emitting tube. However, if a first halide is sealed together withthe iron halide, the strong emission of the ultraviolet light has beenfound to be markedly weakened. Further, the ultraviolet light emissionis weakened where an iron halide is used together with another secondhalide.

As described above, the long arc type metal halide discharge lamp of thepresent invention also produces the electrical characteristics and lightemitting characteristics of the discharge lamp substantially equal tothose produced by the conventional discharge lamp having mercury sealedtherein, though the discharge lamp of the present invention issubstantially free from the mercury sealing.

Further, as apparent from samples Nos. 15 and 16, the lamp voltage canbe controlled at a level similar to that of the conventional dischargelamp having mercury sealed therein by sealing together a plurality ofsecond halides containing different metals.

Then, samples Nos. 1 and 2 of the discharge lamps equal in constructionto the discharge lamp according to the fourth embodiment of the presentinvention were tested for the color rendering properties and the colortemperature under the condition that each of these discharge lamps waslit with a lamp power of 350 W, 300 W, 250 W and 200 W, with the resultsas shown in Table 9:

TABLE 9 Sample No. 200W 250W 300W 350W 1 (Prior Art) Color Rendering 3846 54 60 Properties (Ra) Color Temperature (K) 6010 5630 5160 4530 2(Present Invention) Color Rendering 60 61 62 64 Properties (Ra) ColorTemperature (K) 4560 4450 4220 4100

As apparent from Table 9, the color temperature is markedly increasedand the color rendering properties are markedly lowered, with decreasein the lamp power in the conventional discharge lamp (sample No. 1). Inthe discharge lamp of the present invention (sample No. 2), however, thecolor rendering properties and the color temperature remainsubstantially unchanged in spite of the change in the lamp power, makingit possible to perform light control (dimming).

FIG. 14 is a graph showing the spectral distribution of the conventionallong arc type metal halide discharge lamp. On the other hand, FIG. 15 isa graph showing the spectral distribution of the long arc type metalhalide discharge lamp according to the fourth embodiment of the presentinvention. In each of the graphs shown in FIGS. 14 and 15, thewavelength (nm) is plotted on the abscissa, with a relative dischargepower (%) being plotted on the ordinate.

FIG. 14 covers sample No. 1 of the conventional metal halide dischargelamp shown in Table 8. The main bright-line spectra are caused by theelements denoted above the arrows in the drawing. To be more specific,the light emitted from the conventional discharge lamp (sample No. 1)consists mainly of light rays emitted from scandium (Sc), sodium (Na)and mercury (Hg). Since scandium iodide (ScI₃) and sodium iodide (NaI)have a low vapor pressure, the evaporation amounts of these metaliodides are decreased with decrease in the lamp input in theconventional discharge lamp (sample No. 1). On the other hand, mercuryhas a high vapor pressure. As a result, mercury is entirely evaporatedeven if the lamp input is decreased to 200 W. It follows that, if thelamp power is lowered, the light emission from mercury is renderedrelatively predominant, leading to increase in the color temperature.What should be noted is that, if the lamp power is changed for thedimming purpose in the conventional discharge lamp, a large change inthe color temperature is brought about.

When it comes to the metal halide discharge lamp according to the fourthembodiment of the present invention, both sodium and scandium aredecreased with substantially the same rate in accordance with decreasein the lamp power. In addition, since the second halide emits a visiblelight only slightly, the light emitting characteristics of the metalhalide discharge lamp are scarcely affected. It follows that the colortemperature remains substantially constant regardless of decrease in thelamp power. To be more specific, the change in the color temperature wasas large as 1690K in the conventional discharge lamp (sample No. 1) incontrast to only 440K for the discharge lamp according to the fourthembodiment of the present invention, when the lamp power was changedfrom 400 W to 200 W, as apparent from the experimental data given inTables 8 and 9.

Embodiment 7:

FIG. 16 is a front view showing a cross section of a metal halidedischarge lamp according to a fifth embodiment of the present invention.As shown in the drawing, the discharge lamp comprises a hermetic vessel51, a pair of electrodes 52, a heat insulating means 53, an outer tube54, a support band 55, a base 56, and a conductor wire 57. The hermeticvessel 51, which is made of a quartz glass, has an inner diameter of 12mm. Pinch sealing portions 51 a are formed on both end portions of thehermetic vessel 51. The electrode 52 is positioned in a central portionof the small diameter portion at the end of the hermetic vessel 51. Thesubstrate portion of the electrode 52 is buried in the pinch sealingportion 51 a so as to permit the electrode 52 to be fixed to thehermetic vessel 51. Further, the distance between the two electrodes 52is set at 17 mm.

The heat insulating means 53 is arranged to cover the outer surface ofthat region of the hermetic vessel 51 which surrounds the electrode 52.The outer tube 54 comprises a quartz glass cylinder. The both open endsof the cylinder are sealed by pinch sealing portions 54 a. The hermeticvessel 51 is housed in the outer tube 54 with the support bands 55, 55interposed therebetween such that a relatively small free space isformed within the outer tube 54. The base 56 is fixed to the pinchsealing portion 54 a by using a base cement. Further, the pinch sealingportion 54 a of the outer tube 54 is electrically connected to the pinchsealing portion 51 a of the hermetic vessel 51 by the conductor wire 57.

Sealed in the hermetic vessel 51 were 1.5 mg of scandium iodide as afirst halide, 7.5 mg of sodium iodide as a first halide, 2.66×10³ Pa ofargon as a rare gas, and 5 mg of the second halide shown in Table 10 soas to prepare a metal halide discharge lamp constructed as describedabove. For comparison, also prepared was a conventional metal halidedischarge lamp of the same construction, except that 12.5 mg of mercurywas sealed in the hermetic vessel 51 in place of the second halidesealed in the discharge lamp of the present invention. Each of thedischarge lamps thus prepared was lit at a lamp input of 100 W so as toevaluate the lamp voltage, light emitting efficiency, color temperature,and general color rendering index Ra, with the results as shown in Table10:

TABLE 10 Color Light rendering Second Lamp emitting properties ColorSample No. halide voltage efficiency (Ra) temperature 1 — 122 V 71 lm/W61 4120K (Prior Art) 2 AlI₃ 112 V 67 lm/W 65 4140K 3 FeI₂ 110 V 66 lm/W67 4480K 4 ZnI₂ 111 V 68 lm/W 64 4160K 5 SbI₃ 106 V 63 lm/W 68 4140K 6MnI₂ 80 V 66 lm/W 64 4250K 7 CrI₂ 109 V 66 lm/W 68 4230K 8 GaI₃ 115 V 67lm/W 67 4180K 9 CoI₂ 110 V 65 lm/W 66 4380K 10 NiI₂ 105 V 65 lm/W 684460K

As apparent from Table 10, the metal halide discharge lamp according tothe fifth embodiment of the present invention, in which mercury is notsealed in the hermetic vessel, produces the electrical and lightemitting characteristics substantially equal to those produced by theconventional metal halide discharge lamp having mercury sealed in thehermetic vessel.

Let us describe the pressure during the lighting of the discharge lampof the fifth embodiment with reference to sample No. 2 (presentinvention) and sample No. 1 (prior art).

First of all, the pressure during the lighting of the discharge lamp ofsample No. 1 (prior art) is proportional to the amount of mercury (thenumber of mols). Likewise, the pressure during the lighting of thedischarge lamp of sample No. 2 (present invention) is proportional tothe amount of aluminum iodide (AlI₃). In terms of the number of mols,the ratio of the mercury amount in sample No. 1 to the aluminum iodideamount in sample No. 2 is about 5:1. It follows that the ratio of sampleNo. 1 to sample No. 2 in terms of the pressure during the lighting ofthe discharge lamp is also about 5:1. Since the estimated pressure forsample No. 1 is about 15 atmospheres, the pressure for sample No. 2 isabout 3 atmospheres.

The metal halide discharge lamp gives rise to a problem that the quartzglass forming the light emitting tube reacts with the halide, with theresult that the quartz glass is rendered brittle during the use of thedischarge lamp over a long period of time. It follows that the lightemitting tube is rendered incapable of withstanding the internalpressure, leading to breakage of the light emitting tube.

What should be noted in this connection is that the light emitting tubeis substantially free from mercury in the discharge lamp of the presentinvention. It follows that the internal pressure of the light emittingtube is held low during the lighting of the discharge lamp. Naturally,the danger of the light emitting tube breakage can be markedlysuppressed in the present invention.

Further, sample No. 2 of the metal halide discharge lamp according tothe fifth embodiment of the present invention was tested together withsample No. 1 for the prior art for the rising of the spectralcharacteristics. Used in this experiment was a lighting circuit whichpermits the rising of the light flux to become 100% in 8 seconds afterthe switching on. The spectral distribution of the visible light emittedfrom samples Nos. 1 and 2 of the discharge lamps was measured everysecond after the switching on by using an instant spectroscope, andchromaticity coordinates for each second were calculated on the basis ofthe spectral distribution thus measured.

FIG. 17 is a graph of chromaticity showing the rising of the spectralcharacteristics for the metal halide discharge lamp according to thefifth embodiment of the present invention and for the conventional metalhalide discharge lamp. In the graph of FIG. 17, the x-axis of thechromaticity coordinates is plotted on the abscissa, with the y-axis ofthe chromaticity coordinates being plotted on the ordinate. The regionsurrounded by a frame of solid line in FIG. 17 represents a white regionfor a headlamp for an automobile, which is specified in JapaneseIndustrial Standards (JIS). Curve C in FIG. 17 represents the rising ofthe spectral distribution for sample No. 2 of the metal halide dischargelamp according to the fifth embodiment of the present invention. On theother hand, curve D denotes the rising of the spectral characteristicsfor sample No. 2 of the conventional discharge lamp.

Curve D in the graph clearly shows that the spectral characteristics forthe conventional discharge lamp (sample No. 1) were poor in the initialstage, failing to fall within the white region specified in JIS. This isbecause mercury alone emits light in the conventional discharge lamp. Ittook about one minute for the spectral characteristics to fall withinthe white region.

On the other hand, the spectral characteristics for the sample No. 2 ofthe discharge lamp of the present invention fell within the white regionimmediately after the switching on. This is because light was emittedfrom both sodium and scandium. It follows that the metal halidedischarge lamp according to the fifth embodiment of the presentinvention is adapted for use in the field in which are required bothprompt rising of the light flux and prompt rising of the spectralcharacteristics after the switching on.

Embodiment 8:

FIG. 18 is a cross sectional view showing a gist portion of a metalhalide discharge lamp according to a sixth embodiment of the presentinvention. As shown in the drawing, the discharge lamp comprises ahermetic vessel 61, a pair of electrodes 62, a base 63, and an outerlead wire 64. The hermetic vessel 61 is made of a quartz glass and hasan elliptical cross section. The largest inner diameter of the hermeticvessel 61 is 32 mm. Slender sealing portions 61 a extend outward fromboth ends of the hermetic vessel 61. The hermetic vessel 61 is sealedwithin the sealing portion 61 a with a sealing metal foil made ofmolybdenum arranged within the sealing portion 61 a. An electric currentis supplied to the electrode 62 through the molybdenum foil. Theelectrode 62 consists of an electrode shaft 62 a and a coil 62 b. Theproximal end portion of the electrode shaft 62 a is buried in andsupported by the sealing portion 61 a. In this embodiment, the distancebetween the two electrodes 62, 62 is set at 30 mm. The base 63 ismounted to an end portion of the sealing portion 61 a by a base cement,and the outer lead wire 64 extends to the outside through a hole formedalong the axis of the sealing portion 61 a. The outer lead wire 64,which is covered with an insulating film, is provided with a connectionterminal 64 a at the tip.

Prepared was a metal halide discharge lamp by sealing in the hermeticvessel 61 first halides consisting of 4 mg of dysprosium bromide(DyBr₃), 4 mg of holmium bromide (HoBr₃), and thulium bromide (TmBr₃).Also sealed were 1.33×10⁴ Pa of argon as a rare gas and 30 mg of asecond halide shown in Table 11.

A conventional metal halide discharge lamp was also prepared similarlyfor the purpose of comparison, except that 90 mg of mercury was sealedin the hermetic vessel 61 in place of the second halide sealed in thedischarge lamp of the present invention.

The metal halide discharge lamps including the conventional dischargelamp thus prepared were lit under a constant input power of 2 kW forevaluation of the lamp voltage, light emitting efficiency, colortemperature and general color rendering index (Ra), with the results asshown in Table 11:

TABLE 11 Color Light rendering Sample Second Lamp emitting propertiesColor No. halide voltage efficiency (Ra) temperature 1 — 116 V 94 lm/W91 5120K (Prior Art) 2 AlI₃ 104 V 92 lm/W 92 5020K 3 FeI₂ 107 V 93 lm/W90 5220K 4 ZnI₂ 112 V 92 lm/W 92 5340K 5 SbI₃ 106 V 89 lm/W 92 5080K 6CrI₂ 109 V 90 lm/W 91 5020K 7 GaI₃ 115 V 90 lm/W 89 5220K 8 ZrI₄ 116 V88 lm/W 93 5430K

As apparent from Table 11, it has been confirmed that the metal halidedischarge lamp according to the sixth embodiment of the presentinvention exhibits the electrical and light emitting characteristicssubstantially equal to those produced by the conventional metal halidedischarge lamp having mercury sealed therein.

Let us describe the pressure during the lighting of the metal halidedischarge lamp according to the sixth embodiment of the presentinvention on the basis of comparison between sample No. 1, shown inTable 11, of the conventional discharge lamp and sample No. 2, shown inTable 11, of the discharge lamp according to the sixth embodiment of thepresent invention. It should be noted that the pressure within thehermetic vessel of the conventional discharge lamp (sample No. 1) isproportional to the mercury amount (the number of mols). Likewise, thepressure within the hermetic vessel of sample No. 2 of the dischargelamp of the present invention is proportional to the amount of aluminumiodide (AlI₃). Since the ratio of the mercury amount (the number ofmols) in sample No. 1 to the aluminum iodide amount in sample No. 2 is6:1, the pressure within the hermetic vessel in sample No. 2 during thelighting of the discharge lamp is ⅙ of that in sample No. 1. It shouldbe noted that the pressure within the hermetic vessel during thelighting of the conventional discharge lamp of sample No. 1 is estimatedat 12 atmospheres. It follows that the pressure within the hermeticvessel for sample No. 2 of the present invention is about 2 atmospheres.

The metal halide discharge lamp according to the sixth embodiment of thepresent invention is designed to be adapted for use in a projector. Inorder to make the projector as compact as possible, the discharge lampof the sixth embodiment is also designed compact. As a result, the tubewall load is high, leading to a high operating temperature of the lightemitting tube. In a metal halide discharge lamp having a high load, thequartz glass forming the light emitting tube vigorously reacts with thehalide during the lighting over a long period of time. The vigorousreaction causes the quartz glass brittle so as to make the lightemitting tube incapable of withstanding the internal pressure of thelight emitting tube, giving rise to a problem in terms of breakage ofthe light emitting tube.

In the discharge lamp according to the sixth embodiment of the presentinvention, however, the internal pressure of the hermetic vessel is lowduring the lighting of the discharge lamp. It follows that it ispossible to suppress markedly the danger in respect of breakage of thehermetic vessel.

What should also be noted is that the projector is used mainly in anathletic field and, thus, an hot re-starting is required for the metalhalide discharge lamp. For the hot re-starting, it is necessary to applya high pulse voltage to the discharge lamp. In sample No. 1 of theconventional discharge lamp, required was 35 kv of a pulse voltage. Insamples Nos. 2 to 8 of the discharge lamps of the present invention,however, the pulse voltage required for the hot re-starting was as lowas only at most 8 kV because the internal pressure of the hermeticvessel was low during the lighting of the discharge lamp.

Embodiment 9:

Prepared was a metal halide discharge lamp substantially equal inconstruction and size to the discharge lamp shown in FIG. 13. In thisembodiment, however, the discharge medium sealed in the discharge lampwas as follows:

First halide . . . 3 mg of scandium iodide (ScI₃) and 15 mg of sodiumiodide (NaI);

Second halide . . . 20 mg of the halides shown in Table 12;

Third halide . . . 3 mg of cesium iodide (CsI);

Rare gas . . . 2.66×10³ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that cesium iodide was not sealed in the dischargelamp.

Also prepared were conventional metal halide discharge lamps equal tothe discharge lamp of the present invention, except that 40 mg ofmercury was sealed in the discharge lamp. The conventional dischargelamps also include a case where cesium iodide was not sealed in thehermetic vessel.

The discharge lamps thus prepared including the comparative cases andthe conventional discharge lamps were lit at a constant lamp input of400 W for evaluation of the light emitting efficiency and colorrendering properties (general color rendering index Ra), with theresults as shown in Table 12. Incidentally, 5.32×10⁴ Pa of nitrogen gaswas sealed in the outer tube included in any of the discharge lampstested.

TABLE 12 Light Color emitting rendering Second efficiency propertiesSample No. CsI halide (lm/W) (Ra) 1 a . . . none — 101 62 (Prior Art) b. . . sealed — 98 61 2 a . . . none AlI₃ 96 65 b . . . sealed ″ 106 67 3a . . . none ZnI₂ 94 68 b . . . sealed ″ 108 70 4 a . . . none GaI₃ 9767 b . . . sealed ″ 107 70

As apparent from Table 12, the light emitting efficiency is somewhatlowered by the cesium iodide sealing in the conventional metal halidedischarge lamp having mercury sealed therein.

Also, in the comparative cases where cesium iodide was not sealed, i.e.,case “a” for each of samples Nos. 2 to 4, the light emitting efficiencywas found lower than that for the conventional discharge lamp havingmercury sealed therein.

On the other hand, the discharge lamp of the present invention, i.e.,case “b” for each of samples Nos. 2 to 4, was found to be superior tothe conventional discharge lamp in the light emitting efficiency. Itshould be noted in this connection that, in the conventional dischargelamp, the light is emitted from mercury as well as from sodium andscandium, and that mercury is low in the light emitting efficiency, asdescribed previously, leading to the low light emitting efficiency ofthe conventional discharge lamp. In the discharge lamp according to thisembodiment of the present invention, however, the energy consumed forthe light emission is not distributed to mercury but is distributed tothe light emitting metal alone, with the result that the light emittingefficiency for the present invention is clearly improved, compared withthe prior art.

Further, the discharge lamp according to the present invention issomewhat superior to the comparative case, i.e., case “a” for samplesNos. 2 to 4, in the color rendering properties, too.

Embodiment 10:

Prepared was a metal halide discharge lamp substantially equal inconstruction to the discharge lamp shown in FIG. 13. In this embodiment,however, the light emitting tube of the discharge lamp had an innerdiameter of 12 mm, and the distance between the two electrodes was 17mm. Further, the discharge medium sealed in the discharge lamp was asfollows:

First halide . . . 1.5 mg of scandium iodide (ScI₃) and 17.5 mg ofsodium iodide (NaI);

Second halide . . . 5 mg of the halides shown in Table 13;

Third halide . . . 1.5 mg of cesium iodide (CsI);

Rare gas . . . 2.66×10³ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that cesium iodide was not sealed in the dischargelamp.

Also prepared were conventional metal halide discharge lamps equal tothe discharge lamp of the present invention, except that 12.5 mg ofmercury was sealed in the discharge lamp. The conventional dischargelamps also include a case where cesium iodide was not sealed in thelight emitting tube.

The discharge lamps thus prepared including the comparative cases andthe conventional discharge lamps were lit at a constant lamp input of100 W for evaluation of the light emitting efficiency and colorrendering properties (general color rendering index Ra), with theresults as shown in Table 13. Incidentally, 5.32×10⁴ Pa of nitrogen gaswas sealed in the light emitting tube included in any of the dischargelamps tested.

TABLE 13 Light Color emitting rendering Second efficiency propertiesSample No. CsI halide (lm/W) (Ra) 1 a . . . none — 71 61 (Prior Art) b .. . sealed — 69 60 2 a . . . none AlI₃ 67 65 b . . . sealed ″ 77 66 3 a. . . none NiI₂ 65 68 b . . . sealed ″ 76 68 4 a . . . none MnI₂ 68 64 b. . . sealed ″ 77 63

The tendencies similar to those for Embodiment 9 were also recognized inthis Embodiment 10, too.

Embodiment 11:

Prepared was a metal halide discharge lamp substantially equal inconstruction to the discharge lamp shown in FIG. 13. In this embodiment,however, the light emitting tube of the discharge lamp had an innerdiameter of 25 mm, and the distance between the two electrodes was 60mm. Further, the discharge medium sealed in the discharge lamp was asfollows:

First halide . . . 12 mg of dysprosium iodide (DyI₃) and 3 mg ofthallium iodide (TlI);

Second halide . . . 40 mg of the halides shown in Table 14;

Third halide . . . 15 mg of cesium iodide (CsI);

Rare gas . . . 2.39×10³ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that cesium iodide was not sealed in the dischargelamp.

Also prepared were conventional metal halide discharge lamps equal tothe discharge lamp of the present invention, except that 150 mg ofmercury was sealed in the discharge lamp. The conventional dischargelamps also include a case where cesium iodide was not sealed in thelight emitting tube.

The discharge lamps thus prepared including the comparative cases andthe conventional discharge lamps were lit at a constant lamp input of 1kW for evaluation of the light emitting efficiency and color renderingproperties (general color rendering index Ra), with the results as shownin Table 14. Incidentally, 5.32×10⁴ Pa of nitrogen gas was sealed in theouter tube included in any of the discharge lamps tested.

TABLE 14 Light Color emitting rendering Second efficiency propertiesSample No. CsI halide (lm/W) (Ra) 1 a . . . none — 81 92 (Prior Art) b .. . sealed — 80 93 2 a . . . none AlI₃ 74 92 b . . . sealed ″ 88 93 3 a. . . none SbI₃ 76 91 b . . . sealed ″ 87 92 4 a . . . none FeI₂ 75 92 b. . . sealed ″ 86 92

The tendencies similar to those for Embodiments 9 and 10 were alsorecognized in this Embodiment 11, too.

Embodiment 12:

Prepared was a metal halide discharge lamp substantially equal inconstruction to the discharge lamp shown in FIG. 18. In this embodiment,however, the light emitting tube of the discharge lamp had an innerdiameter of 32 mm, and the distance between the two electrodes was 30mm. Further, the discharge medium sealed in the discharge lamp was asfollows:

First halide 4 mg of dysprosium bromide (DyBr₄), 4 mg of holmium bromide(HoBr₃) and 4 mg of thulium bromide (TmBr₃);

Second halide . . . 30 mg of the halides shown in Table 15;

Third halide . . . 5 mg of cesium iodide (CsI);

Rare gas . . . 1.33×10⁴ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that cesium iodide was not sealed in the dischargelamp.

Also prepared were conventional metal halide discharge lamps equal tothe discharge lamp of the present invention, except that 90 mg ofmercury was sealed in the discharge lamp. The conventional dischargelamps also include a case where cesium iodide was not sealed in thelight emitting tube.

The discharge lamps thus prepared including the comparative cases andthe conventional discharge lamps were lit at a constant lamp input of 2kW for evaluation of the light emitting efficiency and color renderingproperties (general color rendering index Ra), with the results as shownin Table 15.

TABLE 15 Light Color emitting rendering Second efficiency propertiesSample No. CsI halide (lm/W) (Ra) 1 a . . . none — 94 91 (Prior Art) b .. . sealed — 93 92 2 a . . . none AlI₃ 87 92 b . . . sealed ″ 101 93 3 a. . . none MnI₂ 86 90 b . . . sealed ″ 100 92 4 a . . . none FeI₂ 88 92b . . . sealed ″ 102 93

The tendencies similar to those for Embodiments 9 to 11 were alsorecognized in this Embodiment 12, too.

Embodiment 13:

Prepared was a metal halide discharge lamp substantially equal inconstruction to the discharge lamp shown in FIG. 13. In this embodiment,however, the light emitting tube of the discharge lamp had an innerdiameter of 20 mm, and the distance between the two electrodes was 42mm. Further, the inner space of the outer tube 49 was held at a vacuumcondition of 1.33×10⁻² Pa or less. Still further, the discharge mediumsealed in the discharge lamp was as follows:

First halide . . . 3 mg of scandium iodide (ScI₃) and 15 mg of sodiumiodide (NaI);

Second halide . . . 20 mg of the halides shown in Table 16;

Rare gas . . . 2.66×10³ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that 5.32×10⁴ Pa of nitrogen gas was sealed in theouter tube 49 of the discharge lamp.

Also prepared were conventional metal halide discharge lamps equal tothe discharge lamp of the present invention, except that 40 mg ofmercury was sealed in the light emitting tube 41. The conventionaldischarge lamps also include a case where nitrogen gas was not sealed inthe outer tube 49.

The discharge lamps thus prepared including the comparative cases andthe conventional discharge lamps were lit at a constant lamp input of400 W for evaluation of the light emitting efficiency and colorrendering properties (general color rendering index Ra), with theresults as shown in Table 16.

TABLE 16 Light Color Inner space emitting rendering of outer Secondefficiency properties Sample No. tube halide (lm/W) (Ra) 1 a . . . N₂gas — 101 62 (Prior Art) b . . . vacuum — 103 63 2 a . . . N₂ gas AlI₃96 65 b . . . vacuum ″ 106 67 3 a . . . N₂ gas FeI₂ 95 68 b . . . vacuum″ 107 70 4 a . . . N₂ gas GaI₃ 97 67 b . . . vacuum ″ 108 69

As apparent from Table 16, in sample No. 1 of the conventional dischargelamp having mercury sealed therein, an appreciable difference in any ofthe light emitting efficiency and the color rendering properties Ra isnot recognized between case “a ” where a nitrogen gas was sealed in theouter tube and case “b” where the inner space of the outer tube was heldat a vacuum condition.

On the other hand, in any of samples Nos. 2 to 4 of the discharge lampsin which mercury was not sealed, the light emitting efficiency for case“b” where the inner space of the outer tube was held at a vacuumcondition was clearly higher than that for case “a” where a nitrogen gaswas sealed in the outer tube. In other words, the discharge lamp of thepresent invention in which the inner space of the outer tube is held ata vacuum condition is clearly advantageous in the light emittingefficiency over the conventional discharge lamp. It should be noted inthis connection that mercury also emits light in the conventionaldischarge lamp in addition to the light emission from sodium andscandium, and that mercury is low in the light emitting efficiency, asdescribed previously. In the present invention, however, all the lightemitting energy is distributed to the light emitting metals contained inthe first halides, leading to the high light emitting efficiency.Further, the discharge lamp according to Embodiment 13 was foundsomewhat superior to the conventional discharge lamp in the colorrendering properties, too.

The tendencies similar to those for Embodiments 9 to 11 were alsorecognized in this Embodiment 13, too.

Embodiment 14:

Prepared was a metal halide discharge lamp substantially equal inconstruction to the discharge lamp shown in FIG. 13. In this embodiment,however, the light emitting tube of the discharge lamp had an innerdiameter of 12 mm, and the distance between the two electrodes was 17mm. Further, the inner space of the outer tube 49 was held at a vacuumcondition of 1.33×10⁻² Pa or less. Still further, the discharge mediumsealed in the discharge lamp was as follows:

First halide . . . 1.5 mg of scandium iodide (ScI₃) and 7.5 mg of sodiumiodide (NaI);

Second halide . . . 5 mg of the halides shown in Table 17;

Rare gas . . . 2.66×10³ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that 5.32×10⁴ Pa of nitrogen gas was sealed in theouter tube 49 of the discharge lamp.

Also prepared were conventional metal halide discharge lamps equal tothe discharge lamp of the present invention, except that 12.5 mg ofmercury was sealed in the light emitting tube 41. The conventionaldischarge lamps also include a case where nitrogen gas was not sealed inthe outer tube 49 to keep at a vacuum condition the inner space of theouter tube 49. The discharge lamps thus prepared including thecomparative cases and the conventional discharge lamps were lit at aconstant lamp input of 100 W for evaluation of the light emittingefficiency and color rendering properties (general color rendering indexRa), with the results as shown in Table 17.

TABLE 17 Light Color Inner space emitting rendering of outer Secondefficiency properties Sample No. tube halide (lm/W) (Ra) 1 a . . . N₂gas — 71 61 (Prior Art) b . . . vacuum — 74 64 2 a . . . N₂ gas AlI₃ 6765 b . . . vacuum ″ 77 67 3 a . . . N₂ gas NiI₂ 65 68 b . . . vacuum ″76 70 4 a . . . N₂ gas ZnI₂ 68 64 b . . . vacuum ″ 79 66

The tendencies similar to those for Embodiments 13 were also recognizedin this Embodiment 14, too.

Embodiment 15:

Prepared was a metal halide discharge lamp substantially equal inconstruction to the discharge lamp shown in FIG. 13. In this embodiment,however, the light emitting tube of the discharge lamp had an innerdiameter of 25 mm, and the distance between the two electrodes was 60mm. Further, the inner space of the outer tube 49 was held at a vacuumcondition of 1.33×10⁻² Pa or less. Still further, the discharge mediumsealed in the discharge lamp was as follows:

First halide . . . 12 mg of dysprosium iodide (DyI₃) and 3 mg ofthallium iodide (TlI);

Second halide . . . 40 mg of the halides shown in Table 18;

Rare gas . . . 2.39×10³ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that 5.32×10⁴ Pa of nitrogen gas was sealed in theouter tube 49 of the discharge lamp.

Also prepared were conventional metal halide discharge lamps equal tothe discharge lamp of the present invention, except that 150 mg ofmercury was sealed in the light emitting tube 41. The conventionaldischarge lamps also include a case where nitrogen gas was not sealed inthe outer tube 49 to keep at a vacuum condition the inner space of theouter tube 49.

The discharge lamps thus prepared including the comparative cases andthe conventional discharge lamps were lit at a constant lamp input of 1kW for evaluation of the light emitting efficiency and color renderingproperties (general color rendering index Ra), with the results as shownin Table 18.

TABLE 18 Light Color emitting rendering Inner space Second efficiencyproperties Sample No. of outer tube halide (lm/W) (Ra) 1 a . . . N₂ gas— 81 92 (Prior Art) b . . . vacuum — 83 93 2 a . . . N₂ gas AlI₃ 74 92 b. . . vacuum ″ 88 93 3 a . . . N₂ gas SbI₃ 76 91 b . . . vacuum ″ 87 924 a . . . N₂ gas MnI₂ 75 92 b . . . vacuum ″ 86 92

The tendencies substantially equal to those for Embodiments 13 and 14were also recognized in this Embodiment 15, too.

Embodiment 16:

FIG. 19 is a front view showing a metal halide discharge lamp accordingto a seventh embodiment of the present invention. The reference numeralscommon with FIGS. 13 and 19 denote the same members of the dischargelamp and, thus, reference thereto is omitted in the followingdescription. The seventh embodiment shown in FIG. 19 differs from theembodiment shown in FIG. 13 in that the discharge lamp shown in FIG. 19is of a long arc type and is lit by a DC current.

To be more specific, the light emitting tube 41 has an inner diameter of18 mm. A cathode 41 a and an auxiliary electrode 41 b are sealed to oneend of the light emitting tube 41, with an anode 41 c being sealed tothe other end of the tube 41. The cathode 41 a consists of a tungstenrod having a diameter of 1 mm and a length of 15 mm. The tungsten rodcontains thorium, and a tungsten wire having a diameter of 0.4 mm iswound about the tungsten rod. The auxiliary electrode 41 b consists of atungsten wire having a diameter of 0.3 mm. These cathode 41 a, auxiliaryelectrode 41 b and anode 41 c are connected respectively to a sealingfoil 41 d made of molybdenum and hermetically buried inside a sealingportion 41 e. The anode 41 c is connected to a base 50 through thesealing foil 41 e, a conductor wire 48′ and a flare stem 44. Theauxiliary electrode 41 b is connected to the conductor wire 48′ througha starting resistor 45′. The cathode 41 a is connected to the base 50through the conductor wire 48′ and the flare stem 44. The distancebetween the cathode and the anode is set at 40 mm. A heat insulatingfilm 41 f consisting essentially of platinum is formed at an end portionof the light emitting tube 41 on the side of the cathode 41 a. Further,the outer tube 49 consists of a glass tube having an inner diameter of40 mm. The inner space of the outer tube 49 is held at a vacuumcondition.

The discharge medium sealed in the discharge lamp was as follows:

First halide . . . 3 mg of scandium iodide (ScI₃) and 15 mg of sodiumiodide (NaI);

Second halide . . . 20 mg of the halides shown in Table 19;

Rare gas . . . 3.72×10³ Pa of argon gas.

For comparison, a conventional metal halide discharge lamp was similarlyprepared, except that 40 mg of mercury was sealed in the light emittingtube 41 of the discharge lamp.

Five discharge lamps were prepared for each of samples Nos. 2 to 4 ofthe seventh embodiment of the present invention. Also three dischargelamps were prepared for sample No. 1 of the prior art. The dischargelamps thus prepared including the conventional discharge lamps were litby a DC current at a constant rated output of 360 W for evaluation ofthe light emitting efficiency (lm/W), lamp voltage (V), colortemperature (K), and color rendering properties (general color renderingindex Ra), with the results as shown in Table 19.

TABLE 19 Color Light rendering Second Lamp emitting properties ColorSample No. halide voltage efficiency (Ra) temperature 1 — 132 V 101 lm/W62 4320K (Prior Art) 2 AlI₃ 112 V 96 lm/W 65 4120K 3 ZnI₂ 120 V 98 lm/W65 4160K 4 GaI₃ 125 V 97 lm/W 67 4130K

As apparent from Table 19, the metal halide discharge lamp according tothe seventh embodiment of the present invention, which is somewhatinferior in the light emitting efficiency to the conventional dischargelamp, is fully comparable with the conventional discharge lamp in theother characteristics of the discharge lamp.

Then, sample No. 3 of the discharge lamp of the present invention andsample No. 1 of the conventional discharge lamp were tested for thecolor rendering properties and the color temperature under the conditionthat each of these discharge lamps was lit with an input power of 200 W,250 W and 300 W, with the results as shown in Table 20:

TABLE 20 Sample No. 200W 250W 300W 1 (prior Art) Color Rendering 38 4657 Properties (Ra) Color Temperature (K) 6010 5680 5210 3 (PresentInvention) Color Rendering 59 62 63 Properties (Ra) Color Temperature(K) 4500 4210 4150

As apparent from Table 20, the color rendering properties Ra was loweredand the color temperature (K) was increased in the conventionaldischarge lamp with decrease of the lamp input from the rated value. Inthe discharge lamp of the present invention, however, the changes in thecolor rendering properties Ra and the color temperature (K) weremaintained substantially constant in spite of the change in the inputpower, supporting that the discharge lamp of the present invention issuitable for the dimming.

Further, each of the conventional discharge lamp and the discharge lampsof the present invention was subjected to a horizontal lighting underthe condition that each lamp was kept lit for 2 hours at 400 W, whichwas 10% higher than the rated power, followed by keeping the light putout for 10 minutes, so as to look into the occurrence of rupture of thelight emitting tube and the hot re-starting voltage after put-out of thelight for 2 seconds during the lighting at a rated input power. Ruptureof the light emitting tube was not found at all even after the lightingfor about 2,500 hours. On the other hand, the average values of the hotre-starting voltage were as shown in Table 21:

TABLE 21 Re-starting Sample No. Second Halide Voltage (kV) 1 (Prior Art)— 1.8 2 AlI₃ 0.89 3 ZnI₂ 0.8 4 GaI₃ 1.0

As apparent from Table 21, the re-starting voltage for the dischargelamp of the present invention is markedly lower than that for theconventional discharge lamp.

Further, a severe color separation was observed during the lighting ofthe conventional discharge lamp. On the other hand, a color separationwas observed only slightly in the discharge lamp of the presentinvention. In this case, the discharge lamp was considered to besufficient for being put to practical use.

Embodiment 17:

Prepared was a metal halide discharge lamp equal in construction andsize to the discharge lamp shown in FIG. 8 and adapted for use in aheadlamp for a vehicle. The discharge medium sealed in the dischargelamp was as follows:

First halide . . . 0.14 mg of scandium iodide (ScI₃) and 0.7 mg ofsodium iodide (NaI);

Second halide . . . 0.4 mg of the halides shown in Table 22;

Rare gas . . . 5 atmospheres of xenon gas.

For comparison, a conventional metal halide discharge lamp was preparedby further sealing 1 mg of mercury in addition to the second halide.

Each of the discharge lamps of the present invention and theconventional discharge lamp thus prepared was lit at a constant lampinput of 35 W for evaluation of the lamp voltage, light emittingefficiency, color rendering properties (general color rendering indexRa), and color temperature, with the results as shown in Table 22.

TABLE 22 Color Light rendering Second Lamp emitting properties ColorSample No. halide voltage efficiency (Ra) temperature 1 — 83 V 87 lm/W63 4120K (Prior Art) 2 AlI 65 V 81 lm/W 68 3960K 3 FeI₂ 70 V 79 lm/W 714210K 4 ZnI₂ 75 V 81 lm/W 65 3830K 5 MnI₂ 66 V 81 lm/W 65 4230K 6 GaI₃76 V 78 lm/W 65 4330K

The lamp voltage is determined by the sealing amount of mercury in theconventional discharge lamp. In the discharge lamp of the presentinvention, however, the lamp voltage is determined by the evaporationamount of the second halide. It follows that a desired lamp voltage canbe obtained without difficulty in the discharge lamp of the presentinvention by applying a satisfactory heat insulation to the lightemitting tube. As apparent from Table 22, the lamp voltage for thedischarge lamp of the present invention was found somewhat lower thanthat for the conventional discharge lamp. However, since the lampvoltage was higher than 50V, the discharge lamp of the present inventiondoes not give rise to any practical problem because an electroniclighting circuit is used for lighting a small metal halide dischargelamp of this type. Also, the discharge lamp of the present invention issomewhat inferior to the conventional discharge lamp in the lightemitting efficiency. However, the discharge lamp of the presentinvention tends to improve the color rendering properties because avisible light is slightly emitted by the metal contained in the secondhalide such as aluminum.

FIG. 20 is a graph showing the changes in the chromaticity in respect ofsample No. 2 of the discharge lamp of the present invention and sampleNo. 1 of the conventional discharge lamp shown in Table 22.

The graph shows the range of chromaticity of white color specified inJIS (Japanese Industrial Standards) D 5500-1984 relating to the lampsfor automobiles. Curve E shown in the graph denotes the changes in thechromaticity for the discharge lamp of the present invention, with curveF denoting the changes in the chromaticity for the conventionaldischarge lamp. The numbers put near the measuring points for each ofthese curves E and F denote the time (seconds) after the lighting of thedischarge lamp. Used in this experiment was a lighting circuit designedsuch that 2.6A of lamp current was allowed to flow immediately after theturning on of the power source, followed by gradually decreasing thecurrent so as to control the lamp power at a rated power of 35 W.

As apparent from FIG. 20, the discharge lamp of the present inventionemits light falling within a white color range in 0.5 second or lessafter the lighting; whereas, the conventional discharge lamp emits lightfalling within a white color range in 18 seconds after the lighting.

Further, each of sample No. 2 of the discharge lamp of the presentinvention and sample No. 1 of the conventional discharge lamp was lit ata lamp input power of 15 W, 20 W, 25 W and 30 W for evaluation of thegeneral color rendering index Ra and the color temperature (K), with theresults as shown in Table 23:

TABLE 23 Sample No. 15W 20W 25W 30W 1 (Prior Art) Color Rendering 40 4558 61 Properties (Ra) Color Temperature (K) 5640 4970 4630 4350 2(Present Invention) Color Rendering 63 64 65 66 Properties (Ra) ColorTemperature (K) 4280 4220 4110 4040

Sample No. 1 of the conventional discharge lamp has a high mercury vaporpressure, with the result that mercury is entirely in an evaporatedstate even if the lamp input power is lowered to 15 W. It follows thatthe light emission from mercury is rendered predominant with decrease inthe lamp input power so as to increase the color temperature and tolower the color rendering properties. It may be of no difficulty tounderstand that the conventional discharge lamp is unsuitable for thedimming in the practical sense.

On the other hand, both the color rendering properties and the colortemperature remain substantially unchanged regardless of change in thelamp input power in the sample No. 2 of the discharge lamp of thepresent invention. Clearly, the discharge lamp of the present inventionis sufficiently adapted for the dimming.

Further, the re-starting voltage in the step of hot re-starting of thedischarge lamp was measured for each of the discharge lamps of thepresent invention and the conventional discharge lamp, with the resultsas shown in Table 24. For measuring the re-starting voltage, thedischarge lamp was kept lit for 30 minutes, followed by putting out thelamp and, then, re-starting the lighting of the lamp. It should be notedthat, if the discharge lamp is kept put out for a longer time, theelectrode temperature is lowered so as to make it difficult for thedischarge lamp to be re-started. On the other hand, the vapor pressuresof mercury and the metal halide within the light emitting tube arelowered with increase in the time during which the discharge lamp iskept put out, with the result that the re-starting of the discharge lampis facilitated. Because of these contradictory tendencies, it is mostdifficult for the discharge lamp to be re-started in the case where thedischarge lamp is kept put out for about 10 seconds.

TABLE 24 Re-starting Sample No. Second Halide voltage (kV) 1 (Prior Art)— 15.2 2 AlI₃ 8.7 3 FeI₂ 9.1 4 ZnI₂ 9.6 5 MnI₂ 9.3 6 GaI₃ 8.3

The starting voltage for sample No. 1 of the conventional discharge lampwas found to be high because the mercury vapor pressure of the dischargelamp remained to be high.

On the other hand, the vapor pressure of the metal contained in thesecond metal halide was clearly lower than the vapor pressure of mercuryduring the steady lighting time in samples Nos. 2 to 6 of the dischargelamps of the present invention. Nevertheless, the difference between thevapor pressure of the metal contained in the second halide and themercury vapor pressure is minimized in 10 seconds after the dischargelamp is put out. This indicates that the discharge lamp of the presentinvention is quite satisfactory in the re-starting characteristics,compared with the conventional discharge lamp having mercury sealedtherein.

Further, the color characteristics in the vicinity of the electrodeswere measured, covering the case where each of the discharge lamps ofthe present invention and the conventional discharge lamp was lit by aDC current, with the results as shown in Table 25. To be more specific,the color temperatures (K) in the vicinity of the anode and in thevicinity of the cathode were measured, covering the case where the lightemitted from the discharge lamp when the discharge lamp was lit with alamp input power of 35 W was projected onto a screen.

TABLE 25 Color temp. Color temp. Second (K) on (K) on sample No. Halideanode side cathode side 1 (Prior Art) — 5330 3720 2 AlI₃ 4210 3840 3FeI₂ 4420 4010 4 ZnI₂ 4080 3650 5 MnI₂ 4450 4060 6 GaI₃ 4530 4130

As apparent from Table 25, there is a large difference in the colortemperature between the anode side and the cathode side in sample No. 1of the conventional discharge lamp. It is difficult to make up for thelarge difference in the color temperature by designing appropriately theheadlamp using the conventional discharge lamp.

In samples Nos. 2 to 6 of the discharge lamps of the present invention,however, the difference in the color temperature between the anode sideand the cathode side is small. Naturally, the discharge lamp of thepresent invention can be put to practical use satisfactorily.

Embodiment 18:

Prepared was a metal halide discharge lamp substantially equal to thedischarge lamp shown in FIG. 11 and suitable for use in a headlamp of avehicle, except that the end portions of the outer tube 21 werehermetically sealed to the sealing portions 1 a, 1 a of the lightemitting tube 1 and the inner space of the outer tube 21 was held at avacuum condition. The discharge lamp thus prepared was equal to thedischarge lamp of Embodiment 17 in the other construction including thedischarge medium sealed in the light emitting tube. Also prepared was aconventional discharge lamp equal to the conventional discharge lamp ofEmbodiment 17, except that the inner space of the outer tube was held ata vacuum condition.

Each of the discharge lamps of the present invention and theconventional discharge lamp was tested for the lamp voltage (V), lightemitting efficiencies (lm/W), color rendering properties (general colorrendering index) Ra, and color temperature (K), with the results asshown in Table 26.

TABLE 26 Color Light rendering Second Lamp emitting properties ColorSample No. halide voltage efficiency (Ra) temperature 1 — 84 V 89 lm/W63 4010K (Prior Art) 2 AlI₃ 70 V 94 lm/W 68 3890K 3 FeI₂ 76 V 91 lm/W 734120K 4 ZnI₂ 81 V 91 lm/W 68 3720K 5 MnI₂ 71 V 92 lm/W 67 4110K 6 GaI₃80 V 90 lm/W 65 4330K

In the discharge lamp of the present invention, the lamp voltage isincreased and the light emitting efficiency is markedly improved byestablishing a vacuum condition in the inner space of the outer tube. Inthe conventional discharge lamp, however, improvements in the lampvoltage and the light emitting efficiency were achieved only slightly.

Embodiment 19:

Prepared was a metal halide discharge lamp equal in construction andsize to the discharge lamp shown in FIG. 8 and adapted for use in aheadlamp of a vehicle. Discharge medium sealed in the discharge lamp wasas follows:

First halide . . . 0.14 mg of scandium iodide (ScI₃) and 0.7 mg ofsodium iodide (NaI);

Second halide . . . 0.4 mg of zinc iodide (ZnI₂) and 0.1 mg of thehalides shown in Table 27;

Rare gas . . . 5 atmospheres of xenon gas.

Each of the discharge lamps of the present invention thus prepared wastested for the lamp voltage (V), light emitting efficiency (lm/W), colorrendering properties (general color rendering index Ra), and colortemperature (K), with the results as shown in Table 27.

TABLE 27 Color Light rendering Sample Second Lamp emitting propertiesColor No. halide voltage efficiency (Ra) temperature 1 MgI₂ 88 V 81 lm/W65 3890K 2 NiI₂ 91 V 80 lm/W 66 3990K 3 CoI₂ 88 V 82 lm/W 67 4020K 4CrI₂ 96 V 82 lm/W 64 4110K 5 SbI₃ 83 V 79 lm/W 66 3810K 6 ReI₂ 86 V 80lm/W 66 3960K

The second halide, which has a vapor pressure lower than that of mercuryin general, contributes more greatly to formation of the lamp voltagethan mercury under the same vapor pressure.

However, mercury, which always has a high vapor pressure, is evaporatedcompletely when sealed in a small metal halide discharge lamp having asmall load, e.g., rate lamp power of at most 100 W, which is used in,for example, a headlamp for a vehicle. Therefore, the lamp voltage canbe controlled by controlling the sealing amount of mercury.

On the other hand, when it comes to the discharge lamp of the presentinvention in which a second halide is sealed in place of mercury, thevapor pressure of the sealed halide reaches a saturation before thehalide is completely evaporated, making it impossible to furtherincrease the lamp voltage.

However, the lamp voltage can be further increased by sealing aplurality of second halides as in this Embodiment 19. To be morespecific, when the vapor pressure of one of the second halides hasreached a saturation, evaporation of the other second halide contributesto the increase in the lamp voltage. It follows that the lamp voltage inthe case of sealing a plurality of second halides is made higher thanthat in the case of sealing only a single kind of the second halide.

Embodiment 20:

Prepared was a metal halide discharge lamp equal in construction andsize to the discharge lamp shown in FIG. 8 and adapted for use in aheadlamp of a vehicle. Discharge medium sealed in the discharge lamp wasas follows:

First halide . . . 0.14 mg of scandium iodide. (ScI₃) and 0.7 mg ofsodium iodide (NaI);

Second halide . . . 0.4 mg of the halides shown in Table 28;

Third halide . . . 0.1 mg of cesium iodide (CsI);

Rare gas . . . 5 atmospheres of xenon gas.

Also prepared was a conventional metal halide discharge lamp equal tothe discharge lamp of the present invention, except that 1 mg of mercurywas sealed in the discharge lamp in place of the second halide used inthe discharge lamp of the present invention.

Each of the discharge lamps of the present invention and theconventional discharge lamp thus prepared was tested for the lampvoltage (V), light emitting efficiency (lm/W), color renderingproperties (general color rendering index Ra), and color temperature(K), with the results as shown in Table 28.

TABLE 28 Color Light rendering Second Lamp emitting properties ColorSample No. halide voltage efficiency (Ra) temperature 1 — 83 V 86 lm/W63 4140K (Prior art) 2 AlI₃ 63 V 93 lm/W 68 3940K 3 FeI₂ 68 V 92 lm/W 704180K 4 ZnI₂ 73 V 94 lm/W 66 3800K 5 MnI₂ 65 V 94 lm/W 65 4200K 6 GaI₃75 V 92 lm/W 65 4310K

In this embodiment, cesium iodide CsI was added as a third halide.Although the CsI addition did not bring about appreciable changes in thecolor rendering properties Ra and the color temperature, the temperaturedistribution of the arc is flattened by the CsI addition so as tosuppress the heat loss and, thus, to improve the light emittingefficiency. In the prior art having mercury sealed therein, however, animprovement in the light emitting efficiency was not recognized in spiteof the third halide addition.

It should also be noted that, in the present invention, mercury which islow in its light emitting efficiency is not sealed in the light emittingtube so as to make the present invention higher in its light emittingefficiency than the prior art.

Further, an additional experiment was conducted in which the sealingamount of the third halide, i.e., cesium iodide (CsI), was changed insample No. 3 of the discharge lamp of the present invention shown inTable 28 so as to measure the changes in the light emitting efficiency(lm/W), with the results as shown in Table 29:

TABLE 29 CsI (mg) 0.005 0.01 0.05 0.1 LEE* (lm/W) 83 85 88 92 CSI (mg)0.3 0.5 1.0 2.0 2.5 LEE* (lm/W) 91 90 89 84 79 *LEE . . Light EmittingEfficiency

As apparent from Table 29, the CsI addition is effective if the additionamount is 0.01 mg or more. By contraries, if CsI is added excessively,the vapor pressure of the light emitting metal is lowered, with theresult that the light emitting effect is lowered.

Further, the discharge lamps shown in Table 28 were subjected to hot are-starting test under the conditions equal to those in Embodiment 17 soas to measure the re-starting voltage, with the results as shown inTable 30:

TABLE 30 Re-starting Sample No. Second Halide Voltage (kV) 1 (Prior Art)— 15.2 2 AlI₃ 9.2 3 FeI₂ 9.6 4 ZnI₂ 10.1 5 MnI₂ 9.8 6 GaI₃ 8.9

The discharge lamp of the present invention is markedly lower in there-starting voltage than the prior art having mercury sealed in thelight emitting tube, but is somewhat higher in the re-starting voltagethan the case where the third halide of cesium iodide (CsI) is notsealed in the light emitting tube. However, the re-starting voltageshown in Table 30, which is required in the discharge lamp of thepresent invention, does not produce any practical problem.

Embodiment 21:

FIG. 21 is a front view showing a metal halide discharge lamp accordingto an eighth embodiment of the present invention. The reference numeralscommon with FIGS. 8 and 21 denote the same members of the discharge lampand, thus, reference thereto is omitted in the following description.The eighth embodiment shown in FIG. 21 equal to the embodiment shown inFIG. 8 in that the discharge lamp shown in FIG. 21 is suitable for usein a headlamp for a vehicle, but differs from the embodiment shown inFIG. 8 in that the discharge lamp shown in FIG. 21 is lit by a DCcurrent. To be more specific, the discharge lamp shown in FIG. 21comprises a cathode 2 _(K) and an anode 2 _(A).

The hermetic vessel 1, which is prepared from a cylinder having an innerdiameter of 4 mm and a length of 7 mm, is in the shape of a bulb havingan elliptical cross section. Sealing portions 1 a each having a lengthof 30 mm are mounted to the end portions of the hermetic vessel 1. Thecathode 2 _(K) consists of a tungsten rod containing thorium and havinga diameter of 0.4 mm and a length of 6 mm. The proximal end portion ofthe cathode 2 _(K) is welded to one end of a molybdenum foil 3 buried inthe sealing portion 1 a and having a width of 1.5 mm, a length of 15 mmand a thickness of 15 μm. On the other hand, the anode 2 _(A) consistsof a tungsten rod having a diameter of 0.8 mm and a length of 6 mm. Theproximal end portion of the anode 2 _(A) is welded to one end of themolybdenum foil 3. Further, the outer lead wire 4, which consists of aconductive wire having a diameter of 0.5 mm and a length of 25 mm, iswelded to the other end of the molybdenum foil 3.

In preparing the metal halide discharge lamp of the constructiondescribed above, prepared.first are a pair of sealing tubes for formingthe sealing portions la on both end portions of the.hermetic vessel 1.Then, a connection assembly including the cathode 2 _(K), the molybdenumfoil 3 and the outer lead wire 4 is inserted into one of the sealingtubes, followed by fusing under heat the sealing tube using anoxygen-hydrogen burner and subsequently sealing the sealing tube with apinch seal so as to seal the cathode 2 _(K) to the hermetic vessel 1.Further, the first and second halides are sealed in the hermetic vessel1 through the other sealing tube, followed by inserting a connectionassembly including the anode 2 _(A), the molybdenum foil 3 and the outerlead wire 4 into the sealing tube so as to set the distance between theanode and the cathode at 4.2 mm.

In the next step, these connection assemblies are mounted to an exhaustsystem through the sealing tubes so as to exhaust the hermetic vessel 1,followed by introducing a xenon gas at a pressure of 2 atmospheres intothe hermetic vessel 1. Then, the other sealing tube is fused by heatingwith an oxygen-hydrogen burner while cooling the hermetic vessel 1,followed by sealing the anode 2 _(A) to the hermetic vessel 1 with apinch seal, thereby to prepare the metal halide discharge lamp shown inFIG. 21.

The discharge medium sealed in the hermetic vessel 1 included the firstand second halides given below:

First halide . . . 0.17 mg of scandium iodide (ScI₃) and 0.83 mg ofsodium iodide (NaI);

Second halide . . . 0.4 mg of ZnI₂ (sample No. 2 of the discharge lampof the present invention);

. . . 0.2 mg of AlI₃ (sample No. 3 of the discharge lamp of the presentinvention); and

. . . 0.4 mg of FeI₂ (sample No. 4 of the discharge lamp of the presentinvention).

Also prepared was sample No. 1 of the conventional discharge lamp equalto the discharge lamps of the present invention except that 1 mg ofmercury was sealed in the conventional discharge lamp in place of thesecond halide sealed in the discharge lamp of the present invention.

Sample No. 2 of the discharge lamp of the present invention and sampleNo. 1 of the conventional discharge lamp each having a rated lamp inputpower of 35 W were lit with a lamp input power of 20 W, 25 W, 30 W and35 W, so as to measure the light emitting efficiency (lm/W), the colorrendering properties (general color rendering index) Ra, and the colortemperature (K) of the discharge lamps, with the results as shown inTable 31:

TABLE 31 Light Color emitting rendering Color Lamp input efficiencyproperties temperature Sample No. power (W) (lm/W) (Ra) (K) 1 20 45 4970(Prior Art) 35 80 65 4100 2 20 64 4400 (Present Invention) 25 66 4310 3069 4240 35 75 70 4190

FIG. 22 is a graph of chromaticity showing the rising of the spectralcharacteristics of sample No. 2 of the metal halide discharge lampaccording to eighth embodiment of the present invention in comparisonwith sample No. 1 of the conventional discharge lamp. Curve G shown inFIG. 22 represents the rising of the spectral characteristics for thedischarge lamp of the present invention, with curve H denoting therising of the spectral characteristics for the conventional dischargelamp. As apparent from FIG. 22, the light emitted from the dischargelamp of the present invention falls within a range of white lightimmediately after the lighting. On the other hand, the light emittedfrom the conventional discharge lamp falls within a range of white lightabout 18 seconds after the lighting.

Then, an intermittent lighting test was applied to each of the samplesof the discharge lamps under the condition that each lamp was kept litfor 60 minutes at a lamp input power of 42 W, which was 20% higher thanthe rated lamp input power, followed by putting out the lamps for 15seconds in order to look into the rupture of the hermetic vessel. Norupture of the hermetic vessel was found in any of the tested samples1,000 hours after the starting of the test.

Further, the re-starting voltage required for the hot starting 2 secondsafter the putting out of the discharge lamp was measured, with theresults as shown in Table 32:

TABLE 32 Sample No. 1 Prior Art 2 3 4 Re-Starting 12 5 4 4.3 Voltage(kV)

FIG. 23 is a graph showing the relationship between the rare gas sealingpressure (atmospheres) and the rising time of the light flux, coveringthe case where the rare gas sealing pressure was changed in the metalhalide discharge lamp according to the eighth embodiment of the presentinvention shown in FIG. 21. In the graph of FIG. 23, the xenon sealingpressure (atmospheres) is plotted on the abscissa, with the rising time(seconds) of the light flux being plotted on the ordinate.

As seen from FIG. 23, the rising time of the light flux was rapidlyshortened where the sealing pressure of the xenon gas exceeded oneatmosphere, supporting that the discharge lamp of the present inventioncan be put to practical use.

FIG. 24 is a graph showing the relationship between the sealing amount(mg/cc) of ZnI₂ used as a second halide of the present invention and thelamp voltage (V), covering the case where the sealing amount of ZnI₂ waschanged in the metal halide discharge lamp according to the eighthembodiment of the present invention shown in FIG. 21. As seen from FIG.24, a lamp voltage higher than 30V, which is required for lighting thedischarge lamp using an electronic lighting circuit, can be obtained ifthe sealing amount of ZnI₂ exceeds 1 mg/cc.

Embodiment 22:

FIG. 25 is a circuit diagram showing a lighting device of a metal halidedischarge lamp according to a first embodiment of the present invention.The circuit is constructed to permit the metal halide discharge lamp tobe lit by a DC current. As seen from the drawing, the light deviceaccording to the first embodiment of the present invention comprises aDC power source 71, a chopper 72, a control means 73, a lamp currentdetecting means 74, a lamp voltage detecting means 75, a starting means76, and a metal halide discharge lamp 77.

A battery or a rectified DC current source is used as the DC powersource 71. In general, a battery is used as the DC power source 71 inthe case where the metal halide discharge lamp 77 is used in a headlampfor a vehicle. However, it is also possible to use as the DC powersource 71 a rectified DC power source which rectifies an AC current toobtain a DC current. Further, it is possible to connect, as required, anelectrolytic capacitor 71 a in parallel with the DC power source 71 forflattening the DC current.

The chopper 72 serves to convert the DC voltage into a desired voltageand, at the same time, serves to control as desired the metal halidedischarge lamp 77. Where the DC power source 71 has a low voltage, avoltage-increasing chopper is used. By contraries, a voltage-decreasingchopper is used where the DC power source has a high voltage.

The control means 73 serves to control the chopper 72. For example, thecontrol means 73 permits a lamp current at least three times as much asthe rated lamp current to flow from the chopper 72 through the metalhalide discharge lamp 77 immediately after the lighting, followed bygradually decreasing the lamp current with time to reach the rated lampcurrent.

The lamp current detecting means 74, which is inserted in series to thedischarge lamp 77, serves to detect the lamp current so as to supply acontrolled lamp current to the control means 73. On the other hand, thelamp voltage detecting means 75, which is inserted in parallel with thedischarge lamp 77. serves to apply a controlled lamp voltage to thecontrol means 73.

The control means 73, to which the detection signals of the lamp currentand the lamp voltage are fed back, produces a constant power controlsignal so as to control the chopper 72 at a constant power. It should benoted that a micro computer having a time-based control patternincorporated therein in advance is housed in the control means 73 so asto control the chopper 72 such that a lamp current at least three timesas high as the rated lamp current is allowed to flow through the metalhalide discharge lamp immediately after the lighting, followed bydecreasing the lamp current with time.

The starting means 76 is constructed to permit a pulse voltage of 20 kVto be supplied to the metal halide discharge lamp 77 at the startingtime.

According to the lighting device for the metal halide discharge lamp ofthis embodiment, a desired light flux is generated immediately after thelighting, though a DC current is used for the lighting. As a result, thelighting device of the present invention can be used satisfactorily in aheadlamp for a vehicle such as an automobile, which requires rising ofthe light flux in an amount of 25% and 80% of the rated value in 1second and 4 seconds, respectively, after the turning-on of the powersource.

It should be noted that a DC to AC converter is not required in thelighting device of this embodiment, making it possible to achieve a costreduction of about 30%, compared with the lighting device utilizing anAC power source. It is also possible to achieve the weight reduction by15%, leading to a low manufacturing cost of the lighting circuit.

Embodiment 23:

FIG. 26 is a circuit diagram showing a lighting device of a metal halidedischarge lamp according to a second embodiment of the presentinvention. The reference numerals common with FIGS. 25 and 26 denote thesame members of the circuit and, thus, reference thereto is omitted inthe following description. It should be noted that the second embodimentshown in FIG. 26 differs from the first embodiment shown in FIG. 25 inthat the metal halide discharge lamp included in the lighting circuit ofthe second embodiment is lit by an AC current. It should be noted thatthe lighting circuit of the second embodiment includes an AC convertingmeans 78 consisting of a full bridge inverter. To be more specific, apair of series circuits each consisting of a pair of switching means 78a, 78 a are connected in parallel between the output terminals of thechopper 72 so as to form a bridge circuit. Also, an oscillation outputof an oscillator 78 b is alternately supplied to the two diagonallyfacing switching means included in the four switching means 78 a so asto generate a high frequency AC current between the output terminals ofthe bridge circuit. The metal halide discharge lamp 77 is lit by thehigh frequency AC current thus generated.

Embodiment 24:

FIG. 27 schematically shows a headlamp for a vehicle as an illuminationapparatus according to a third embodiment of the present invention.Also, FIG. 28 schematically shows the light distributor portion includedin the headlamp shown in FIG. 27. The headlamp shown in FIGS. 27 and 28comprises a lighting circuit 81, a light distributor 82, a main opticalfiber 83, a light shutter 84, an individual optical fiber 85, and anilluminator 86.

The lighting circuit shown in FIG. 25 or 26 can be used as the lightingcircuit 81. The light distributor 82 includes a case 82 a, alight-collecting reflector 82 b, a metal halide discharge lamp 82 c, andan optical connector 82 d. The light emitted from the metal halidedischarge lamp 82 c is distributed from the portion of the opticalconnector 82 d to the main optical fiber 83. The light distributed fromthe light distributor 82 is transmitted by the main optical fiber 83into the light shutter 84. The light shutter 84 serves to selectivelytransmit the light to each lamp 86 via the individual optical fiber 85.Further, the illuminator 86 consists of a set of a high beam illuminator86 a, a low beam illuminator 86 b and a fog illuminator 86 c. Twoilluminators 86 of the particular construction are mounted to both sideson the front portion of a vehicle such as an automobile.

Embodiment 25:

Prepared was a metal halide discharge lamp adapted for use in Embodiment24 described above. The discharge lamp thus prepared was substantiallyequal in construction to the discharge lamp shown in FIG. 8, except thatthe discharge lamp in this embodiment was designed to have a rated lamppower of 80 W. Also, the distance between the two electrodes was set at2 mm in order to improve the light collecting efficiency. Further, thedischarge medium sealed in the hermetic vessel was as follows:

First halide . . . 0.3 mg of scandium iodide (ScI₃) and 1.5 mg of sodiumiodide (NaI);

Second halide (for sample No. 2) . . . 1 mg of ZnI₂, 1 mg of AlI₃, and 1mg of MnI₂;

Second halide (for sample No. 3) . . . 2 mg of ZnI₂, 1 mg of GaI₃, and 1mg of CrI₂;

Rare gas . . . 5 atmospheres of xenon gas.

Also prepared was a conventional metal halide discharge lampsubstantially equal to the discharge lamp of the present inventionexcept that 15 mg of mercury was sealed in the hermetic vessel in placeof the second halide sealed in the discharge lamp of the presentinvention.

Each of the discharge lamps of the present invention and theconventional discharge lamp thus prepared was lit at a rated input powerof 80 W for evaluation of the lamp voltage (V), light emittingefficiency (lm/W), color rendering properties (general color renderingindex) Ra and the color temperature (K), with the results as shown inTable 33:

TABLE 33 Color Lamp Light rendering Color voltage emitting propertiestemperature Sample No. (V) efficiency (Ra) (K) 1 63 94 lm/W 63 4020(Prior Art) 2 58 88 lm/W 68 3920 3 62 89 lm/W 69 4110

As apparent from Table 33, the discharge lamp of the present inventionproduces characteristics substantially equal to those of theconventional discharge lamp having mercury sealed therein. It shouldalso be noted that, in the system of Embodiment 24, the necessity forchanging the input power for the dimming purpose is increased. In thisrespect, it is highly useful that the dimming can be achieved.

Embodiment 26:

FIG. 29 is a cross sectional view showing a down light as anillumination apparatus according to a fourth embodiment of the presentinvention. As shown in the drawing, the down light comprises a metalhalide discharge lamp 91 and a down light body 92 including a base body92 a, a socket 92 b and a reflecting plate 92 c. The base body 92 a isburied in a ceiling and, thus, provided with a ceiling abutting edge “e”at the lower end. The socket 92 b is mounted to the base body 92 a.Further, the reflecting plate 92 c is supported by the base body 92 aand is positioned to surround the metal halide discharge lamp 91 suchthat the center of the light emission from the discharge lamp 91 ispositioned in substantially the central portion of the reflecting plate92 c.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A metal halide discharge lamp which essentiallypermits disusing mercury, comprising: (a) a refractory andlight-transmitting hermetic vessel; (b) a pair of electrodes fixed tosaid hermetic vessel, the distance between said pair of electrodes being1 to 6 mm; and (c) a discharge medium sealed in the hermetic vessel andcontaining a first halide, a second halide, and Xe gas, (i) said firsthalide being a halide of a metal which achieves a desired lightemission, (ii) said second halide having a relatively high vaporpressure so as to act as a buffer gas to maintain a lamp voltage andcontributing to the lamp voltage more than the first halide, and (iii)said Xe gas being sealed in a pressure of at least one atmosphere. 2.The metal halide discharge lamp according to claim 1, furthercomprising: an outer tube housing a light-emitting tube; and heatinsulating means for suppressing loss of heat generated from thelight-emitting tube.
 3. The metal halide discharge lamp according toclaim 1, wherein said second halide is a halide of at least one metalselected from the group consisting of magnesium, iron, cobalt, chromium,zinc, nickel, manganese, aluminum, antimony, beryllium, rhenium,gallium, titanium, zirconium, and hafnium.
 4. The metal halide dischargelamp according to claim 1, wherein said second halide is based on ahalide of at least one metal selected from the group consisting of iron,zinc, manganese, aluminum and gallium.
 5. The metal halide dischargelamp according to claim 1, wherein said second halide is sealed in anamount of 0.05 to 200 mg/cc of the inner volume of said hermetic vessel.6. A lighting device for a metal halide discharge lamp, comprising: ametal halide discharge lamp defined in claim 1; and a lighting circuitconstructed to supply current in an amount at least three times as muchas a rated lamp current immediately after the lighting of said metalhalide discharge lamp, followed by decreasing the current with time. 7.An illumination apparatus, comprising: an illumination apparatus body;and a metal halide discharge lamp defined in claim 1, said lamp beingsupported by said illumination apparatus body.
 8. A metal halidedischarge lamp which essentially permits disusing mercury, comprising:(a) a refractory and light-transmitting hermetic vessel; (b) a pair ofelectrodes fixed to said hermetic vessel, the distance between said pairof electrodes being 1 to 6 mm; and (c) a discharge medium sealed in thehermetic vessel and containing a first halide, a second halide, and Xegas, (i) said first halide being a halide of at least one metal selectedfrom the group consisting of sodium, scandium, and a rare earth metal,(ii) said second halide having a relatively high vapor pressure so as toact as a buffer gas to maintain a lamp voltage and contributing to thelamp voltage more than the first halide, and (iii) said Xe gas beingsealed in a pressure of at least one atmosphere.
 9. An illuminationapparatus, comprising: an illumination apparatus body; and a metalhalide discharge lamp defined in claim 8, said lamp being supported bysaid illumination apparatus body.
 10. A metal halide discharge lampwhich essentially permits disusing mercury and which is lit by a DCcurrent, comprising: (a) a refractory and light-transmitting hermeticvessel; (b) an anode and a cathode fixed to said hermetic vessel, thedistance between said anode and said cathode being 1 to 6 mm; and (c) adischarge medium sealed in the hermetic vessel and containing a firsthalide, a second halide, and Xe gas, (i) said first halide being ahalide of at least one metal selected from the group consisting ofsodium, scandium, and a rare earth metal, (ii) said second halide havinga relatively high vapor pressure so as to act as a buffer gas tomaintain a lamp voltage and contributing to the lamp voltage more thanthe first halide, and (iii) said Xe gas being sealed in a pressure of atleast one atmosphere.
 11. An illumination apparatus, comprising: anillumination apparatus body; and a metal halide discharge lamp definedin claim 10, said lamp being supported by said illumination apparatusbody.
 12. A metal halide discharge lamp which essentially permitsdisusing mercury and which is used in a headlamp having a rated power ofat most 100 W, comprising: (a) a refractory and light-transmittinghermetic vessel; (b) a pair of electrodes fixed to said hermetic vessel,the distance between said pair of electrodes being 1 to 6 mm; and (c) adischarge medium sealed in the hermetic vessel and containing a firsthalide, a second halide, and Xe gas, (i) said first halide being ahalide of at least one metal selected from the group consisting ofsodium, scandium, and a rare earth metal, (ii) said second halide havinga relatively high vapor pressure so as to act as a buffer gas tomaintain a lamp voltage and contributing to the lamp voltage more thanthe first halide, and (iii) said Xe gas being sealed in a pressure of atleast one atmosphere.
 13. The metal halide discharge lamp according toclaim 12, wherein said second halide is sealed in an amount of 1 to 200mg/cc of the inner volume of said hermetic vessel.
 14. The metal halidedischarge lamp according to claim wherein said hermetic vessel has aninner diameter of 3 to 10 mm and an outer diameter of 5 to 13 mm in thelargest diameter portion.
 15. The metal halide discharge lamp accordingto claim 12, wherein said lamp is lit by a DC current.
 16. The metalhalide discharge lamp according to claim 12, further comprisingultraviolet light-removing means for removing an ultraviolet light fromthe light transmitted to the outside.
 17. A lighting device for a metalhalide discharge lamp, comprising: a metal halide discharge lamp definedin claim 12; and a lighting circuit constructed to supply current in anamount at least three times as much as a rated lamp current immediatelyafter the lighting of said metal halide discharge lamp, followed bydecreasing the current with time.
 18. An illumination apparatus,comprising: an illumination apparatus body; and a metal halide dischargelamp defined in claim 12, said lamp being supported by said illuminationapparatus body.